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
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.
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)
- Birth to 9 Years
- Explosive connectivity, especially short-range connections between nearby brain areas.
- Analogy: like building city streets that connect neighborhoods.
- 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)
- 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)
- 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)
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)
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
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.
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.