When is your brain actually an "adult"?

Summary

Podcast Summary: "When is your brain actually an 'adult'?"

Podcast: Short Wave (NPR)
Host: Regina Barber
Guest: Duncan Astell, neuroscientist, University of Cambridge
Date: January 14, 2026
Length: ~15 minutes

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Overview

This episode tackles the common belief that the brain stops developing and finally becomes "adult" at age 25. Host Regina Barber discusses with neuroscientist Duncan Astell about new scientific findings that challenge this idea. Using a vast dataset of 4,000 MRI scans, Astell explains new research showing that the human brain remains in a process of maturation until around age 32, and that distinct stages mark our brain’s life-long development. The conversation explores what this means for our understanding of adolescence, adult maturity, learning, aging, and what makes the human mind unique compared to other species.

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Key Discussion Points & Insights

The Myth of the "Adult" Brain at 25

• Cultural Belief: Society often points to age 25 as a milestone for adult brain maturity, thought to usher in rational decision-making.
• Personal Reflection: Duncan shares his own experience of feeling "so mature" in his 20s, only to look back with embarrassment—illustrating the fallibility of the 25-view.

  "I thought I was so adult as a 20-something year old...just little things that sort of fill me with embarrassment."
  – Duncan Astell (00:36)

The New Research: Four Stages of Human Brain Development

• Study Overview: Astell’s lab analyzed 4,000 MRI scans from ages birth to 90 to map brain development.
• Discovery: Found four distinct turning points rather than a single "adult" mark at 25.

The Four Stages (03:49 – 07:22)

1. Birth to 9 Years
   • Explosive connectivity, especially short-range connections between nearby brain areas.
   • Analogy: like building city streets that connect neighborhoods.
2. Age 9 to 32
   • Focus shifts from more connections to making networks efficient via long-range "highways"—myelination/insulation aids in rapid information transfer.
   • Analogy: Connecting cities with major highways to facilitate integrated functioning.

     "In that long 9 to 32 phase, what the brain is doing is myelinating, it's insulating those long connections... It's about like efficiency." – Duncan Astell (04:16)

3. Age 32 to 66
   • Marked by stability; the highways are built—the brain’s organizational structure is maintained more than transformed.
   • Analogy: Upkeep rather than expansion or new construction.

   "Up to 66 it's really very stable... that turning point is 66. It's much more gradual and subtle." – Duncan Astell (05:30)

4. 66 and Beyond
   • Changes focus on maintaining the efficiency of the network—key "hubs" grow in importance, reflecting aging-related adaptation.
   • Tasks drawing on well-established connections remain strong; learning new things can become harder.

The “Adolescent” Phase Extends Until 32

• Continuous Change, Not Abrupt Shifts:
  • The period from roughly 9 to 32 is now considered neurodevelopmentally continuous; not saying a 9-year-old and 32-year-old have the same brain, but they’re on a shared developmental trajectory.

  "Adolescence lasts all the way through from nine... till 32, when you reach a sort of adult stable-like plateau." – Duncan Astell (01:48)

• Practical Impact: May explain lingering impulsivity, risk-taking, and social sensitivity in "young adults" well into their twenties.

  "Adolescence is... associated with this time of relative impulsivity, difficulties controlling one's behavior." – Duncan Astell (09:36)

Brain and Mental Health Across the Lifespan (10:18 – 11:19)

• Different Phases, Different Risks:
  • Neurodevelopmental disorders (like ADHD) tend to surface in childhood (6–9 years).
  • Major mental illnesses (depression, schizophrenia) most often first appear in adolescence/early adulthood (before 24–25).
  • Later life stages: dementia and neurodegenerative disorders dominate.

  "75% of [mental] conditions will have their first onset before [you] become 24 or 25." – Duncan Astell (10:48)

What Makes the Human Brain Unique? (11:27 – 13:35)

• "Absurdly Slow" Human Development:

  • Compared to other species, humans take years—even decades—before brain maturation stabilizes, but this is an evolutionary advantage.
  • Prolonged brain and cognitive development maximizes:
    • Adaptation to environment
    • Brain size (grows much after birth)
    • Individual diversity (even identical twins become different)
  • This flexibility and diversity are key to human success.

  "When you have this incredibly long developmental time course, it maximizes adaptation to the environment... It maximizes brain size... It also maximizes diversity across the species." – Duncan Astell (12:21)

• Reassuring Outlook for Parents:

  • The slow process relieves pressure for "early milestones" in childhood.

  "We've got like another like 30.5 years left to go. Let's, you know, chill out... Early and fast is not always good." – Duncan Astell (13:36)

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Memorable Quotes

• On Brain Developmental Trajectories:

  "What we're saying is that the way that the brain changes is relatively consistent through that time period... the brain is attempting to achieve a kind of common goal through that whole period of time."
  – Duncan Astell (08:52)

• On The Uniqueness of Human Brains:

  "If you compare us to other species... human development is so slow, like absurdly slow... but in many ways, I think it's actually the secret of our success as a species."
  – Duncan Astell (11:27)

• On Modern Parenting:

  "I now feel incredibly relaxed at the idea that development's really long. People are on really different timescales. Early and fast is not always good."
  – Duncan Astell (13:36)

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Important Timestamps

• 00:36 – Duncan reflects on perceived adulthood in one's 20s
• 01:48 – New “adolescent” phase up to age 32 explained
• 03:49 – Detailed explanation of four brain development phases
• 05:07 – Analogy to building highways and city infrastructure
• 06:17 – Effects of aging on the brain’s network structure
• 07:22 – Limitations of MRI studies and individual variability
• 09:36 – Implications of prolonged adolescence for behavior/risk
• 10:18 – Mental health disorders and when they tend to emerge
• 11:27 – Evolutionary perspectives on slow human brain development
• 13:36 – Parental and personal reflections on the long timescales

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Tone and Flow

• The conversation is lively, creative, and accessible, with analogies (cities, highways, orchestras) and an encouraging, demystifying tone—suitable for all listeners, regardless of scientific background.
• Both host and guest mix humor with genuine scientific curiosity, leaving the audience with actionable insights and a fresh perspective on human growth and maturity.

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In Summary

The human brain takes much longer to reach adult “maturity” than commonly believed, and its adaptability and prolonged development are key to human flexibility, learning, and resilience. Rather than peaking at 25, our brains follow a much more nuanced, extended, and individual developmental course—one that makes each person unique. Early milestones don’t determine future outcomes, and the slow pace is central to our species' success.

Transcript

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[Regina Barber]

You'Re listening to Shortwave from NPR. There's this idea that something happens in the human brain when, when we turn 25. Suddenly we're developed, we can finally make rational decisions and we might even regret some of our past indiscretions.

[Duncan Astell]

I thought that I was so adult as a 20 something year old. I thought I was so mature. And now I look, even now I look back and I remember various things I said or did not like, big things, actually, but just little things that sort of fill me with embarrassment.

[Regina Barber]

Duncan Astell is a neuroscientist and professor at the University of Cambridge. He and he says that there is some truth to this anecdotal reverence for the age of 25. The cortex of our brain, its outermost layer is filled with neurons.

[Duncan Astell]

So it's kind of computational center for your brain.

[Regina Barber]

It's at its thickest point when we're around three or four years old.

[Duncan Astell]

And then it gradually starts thinning.

[Regina Barber]

Around age 25, the executive control center of your brain, the frontal lobe, is done thinning out. But the question of when the human brain stops developing, well, it was seen.

[Duncan Astell]

Kind of as this sort of unfathomable question. You know, it's this question that neuroscientists.

[Regina Barber]

Hate to be asked until now. A new study from Duncan's lab finds four distinct turning points in human development. And it suggests that we may be in the adolescent phase until we're 32.

[Duncan Astell]

Adolescence lasts all the way through from nine, so just before puberty all the way through till 32, when you reach a sort of adult stable like plateau.

[Regina Barber]

So today on the show, how the human brain develops across our lives and why we might be maturing for a little longer than scientists thought. You're listening to Shortwave, the science podcast from NPR.

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[Regina Barber]

Okay, Duncan, let's talk about your study and how you identified these different turning points. To do this, Your lab analyzed 4,000 MRI scans of human brains from birth to age 90. So what are the development stages you found? What stands out about them?

[Duncan Astell]

So in the very first phase, from zero to nine, what you see is you get this explosion in connectivity. And that is because what the brain is doing is there's a massive proliferation of the connectivity. So it's wiring together closely located areas. So you're forming lots and lots of connections between short range areas. And then from 9 to 32, what the brain is doing is wiring those up. And so you start to get strong myelination of long range connectivity which connects those clusters up together. So in that long 9 to 32 phase, what the brain is doing is it's myelinating, it's insulating those long connections so that it can integrate the information over larger distances. So it's not about, you know, more and more connectivity, it's about like efficiency. Exactly. It's about like an organizational goal that it's trying to achieve. And then essentially it does that all the way up to 32. And then you see this kind of turning point in the data, implying that now the change is coming from something else. And then you get really a much more stable, like pattern.

[Regina Barber]

The more you talk about this, the more I think about cities developing and highways and like from 9 to 32 you're actually connecting those cities. And then from 32 to 66 you have a nice highway. Now it's, everything's connected, but maybe you're maintaining it. I don't know. What would you say then between 32 to 66?

[Duncan Astell]

That's exactly the right way of thinking about it. And it's interesting how lots of different networks, whether they be between your ears, where you drive your car, they adhere to quite similar principles when they're designed. So yeah, up to 66 it's really very stable. And if you look in the data really carefully, you can see that that turning point is 66. It's much more gradual and subtle than the other turning points. And to me, that suggests that it's more variable across individuals. And it's not as clear that it's a completely different mode of change from what you get in the previous section.

[Regina Barber]

Because it might just be about maintaining that highway. Like, if you can just maintain that highway, instead of it deteriorating with age, you might be able to last longer, right?

[Duncan Astell]

Exactly. And into kind of early aging and older aging. We think that those changes with age are primarily about trying to preserve the efficiency of the network. So in early aging, your brain really starts to become more modular, and you can imagine that for lots of activities and tasks, that is still absolutely fine. If you've got to learn something new, where you've got to start to coordinate all sorts of different bits of your brain that you haven't coordinated before in that way, that might be quite tricky. But for doing the things that you know how to do well, that you're doing like 90% of the time, it will still work pretty well. It's a bit like an orchestra that's learnt a piece of music. And once the orchestra knows the piece of music brilliantly well, you don't need the conductor to be there. Everyone knows it like the back of their hand. But try and learn the new piece of music you could really do with the conductor being there to kind of coordinate everything. And then into older aging from kind of 83 onwards, you see that certain key hubs start to play a really crucial role.

[Regina Barber]

Oh, wow. What are the limitations to these MRI scans when it comes to understanding brain development?

[Duncan Astell]

Yeah, that's a great question. So, in a way, I guess what we're looking at is a sort of average brain for each age. So we've got the 4,000 brains, and we've analyzed them all in lots of different ways. But in this sort of final analysis, where we did the kind of machine learning with the turning points, we created an average brain for each year of life. You can imagine that you're smoothing over an awful lot of differences between individuals that you're kind of losing in the data. Now, we know from other studies that things like cardiovascular health and social connectedness are really important for maintaining brain health into older age. Also, we're pulling out one particular aspect of the brain, this idea of the kind of insulation of the wires and using that to look at organization. But there'll probably be a whole host of other changes going on in the brain that are not captured by the type of MRI scan we're doing there. Surely there are kind of hormonal changes, there are changes in the Density of different bits of brain that are all changing as well, that we're not able to look at through this kind of one particular lens that we've chosen.

[Regina Barber]

Yeah, so this adolescent brain, if you think about it, you know, nine year old brain is not the same as a 32 year old brain, like just in the way we live. Why make that one category?

[Duncan Astell]

Yeah, so just to say we didn't decide any of these categories, you know, we were completely agnostic as to whether there would be turning points or where they might be. But the point is that they are there. And you're right, you know, we're not suggesting that a 10 year old's brain and a 30 year old's brain are the same, even though they're in the same category. The way to think about it is that what we're saying is that the way that the brain changes is relatively consistent through that time period. It's continuous, it can be radical and extensive, but the way that it's changing is relatively continuous, which suggests to us that the brain is attempting to achieve a kind of common goal through that whole period of time.

[Regina Barber]

Give me some examples. What do you mean?

[Duncan Astell]

Well, adolescence is sort of associated this time of relative impulsivity, difficulties controlling one's behavior. And when you think of the neuroscience of the brain and the duration with which it takes different brain areas to reach maturity, that might make sense. I mean, I think there are other factors because of course we were just talking about brains here. But there are also massive social changes that are going on during that time. And, and others have shown that actually sometimes that sort of apparent risk taking is actually to do with kind of social norms within groups around the time. But yeah, there are large, large changes happening across that time range.

[Regina Barber]

And what about different brain or mental health conditions at these different developmental points?

[Duncan Astell]

So I think one thing that's interesting that has struck us is that different, you will encounter different types of conditions in these different phases of life. So for example, if you are going to get an ADHD diagnosis at some point in later life, the symptoms will first become recognizable when you're kind of six, seven, eight, nine years of age. Phase two, if you're going to have a mental health condition, your anxieties, depressions, also kind of more severe mental illness like schizophrenia. Adolescence is when you will first have the onset of symptoms. So 75% of all of those conditions will have their first onset before become 24 or 25. And then of course, when you think of the other end of life, you can think of kind of dementia as age related cognitive difficulties, Parkinson's and so on.

[Regina Barber]

Yeah. After everything's said and done, what's your big takeaway about these developmental phases of the human brain?

[Duncan Astell]

Scientifically? It's definitely the slowness. That long process has really struck me and it's got me thinking much more deeply about, Right, what other changes happen in your kind of late 20s, early 30s? We haven't traditionally thought of that as a particularly interesting developmental window because we've assumed, sorry, you're an adult that's 18 or 21 or whatever the law says. If you compare us to other species, for instance, human development is so slow, like absurdly slow. Like when you watch those kind of Attenborough documentaries and the giraffe emerges and within minutes it's up and about, escaping the lions or whatever, whereas human baby arrives and it's useless for years. And you kind of think, what an incredible design flaw. Why on earth would you design a system like that? But in many ways, I think it's actually the secret of our success as a species. Because when you have this incredibly. Yeah, absolutely. Because when you have this incredibly long developmental time course, it maximizes adaptation to the environment. So so much of our development is not spent inside the womb, it's spent outside in the real world. It maximizes brain size. Right. So the baby comes out relatively early on and its brain still has a long way to grow. So overall, our brains are enormous by comparison with what they should be given our body size. And it also maximizes diversity across the species. So because the way that our brains develop is kind of, we call it sort of stochastic, there's randomness in it, which means that each of us gradually moves down a different trajectory. So even if you had, like an identical twin genetically, you would still end up with different brains. And as a species, you could imagine how all of those features, big brains, maximum adaptation, maximum diversity across the species, are all absolute keys to our success.

[Regina Barber]

Wow.

[Duncan Astell]

And so I actually think this idea of, you know, it takes us till we're 32, develop, in a way. I sort of feel like this is kind of our secret weapon as a species relative to other species.

[Regina Barber]

That's fascinating.

[Duncan Astell]

I had never thought of that, personally, I think. I mean, I became a parent about 18 months ago and it is incredibly reassuring to know that whilst other parents are sort of saying, oh, is she walking yet? Is she talking yet? This is of constant drive on, like these kind of early milestones. Yeah, I'm like, guys, we've got like another like 30.5 years left to go. Let's, you know, chill out. We can't keep doing this. So I now feel incredibly relaxed at the idea that development's really long. People are on really different timescales. Early and fast is not always good.

[Regina Barber]

Wow. Thank you, Duncan. This has been really enlightening. Thank you so much for coming on the show.

[Duncan Astell]

My pleasure. Thank you very much for having me.

[Regina Barber]

If you liked this episode, follow us on the NPR app or whatever podcasting platform you're listening for from. Also, you might want to check out our episode on the psychology of why people think things could always be better, or our episode on keeping the brain from aging by doing cognitive workouts. We'll link to them in our episode notes. This episode was produced by Rachel Carlson and edited by our showrunner, Rebecca Ramirez. Tyler Jones checked the facts. Kwesi Lee was the audio engineer. Beth Donovan is our vice president of podcasting and hi, I'm Regina Barber. Thank you for listening to Short Wave from npr.

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[Duncan Astell]

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