When is your brain actually 'adult'?

September 7, 2026

AI Summary

5 min read

When is your brain actually 'adult'?

"Adolescence last all the way through from nine, so just before puberty, all the way through till thirty-two, when you reach a sort of adult stable plateau." That is the central finding from a new study by Duncan Astle, a neuroscientist and professor at the University of Cambridge. The popular idea that the human brain finishes developing at age 25 turns out to be roughly correct—but only as a midpoint, not an endpoint. By analyzing 4,000 MRI scans from birth to age 90, Astle's lab identified four distinct turning points in brain development, and the adolescent phase extends nearly a decade longer than most people assume.

The four phases of brain development

Astle's team used machine learning to analyze the scans agnostically, letting the data reveal where developmental turning points actually occur—without imposing any preconceived categories. The first phase, from birth to age 9, is characterized by an explosion of short-range connectivity. "The brain is wiring together closely located areas," Astle explains, forming the basic infrastructure.

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What you'll learn

1 (00:00) **The Myth of the 25-Year-Old Brain** - The episode introduces the popular idea that the brain finishes developing at age 25, and host Aaron Scott shares his own embarrassment about his past "mature" decisions in his twenties.
2 (01:31) **A New Study Finds Four Brain Development Phases** - Duncan explains that his lab has identified four distinct turning points in brain development, challenging the single "age 25" milestone and suggesting adolescence may last until 32.
3 (03:51) **Phase 1: The Explosion of Connectivity (0-9)** - The brain undergoes a massive proliferation of short-range connections, wiring together closely located areas.
4 (04:14) **Phase 2: The Long-Distance Wiring (9-32)** - The brain shifts to insulating long-range connections through myelination, making communication between distant brain regions more efficient.
5 (05:09) **Phase 3: The Stable Plateau (32-66)** - The brain enters a period of remarkable stability, where the highway system is maintained rather than built.
6 (06:19) **Phase 4: Early and Late Aging (66-90+)** - The brain focuses on preserving network efficiency, becoming more modular and relying on key hubs to coordinate tasks.
7 (07:24) **Limitations of MRI Scans** - The host asks about the limitations of using MRI scans to understand brain development, prompting a discussion of what the study might miss.

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Show Notes

Summer is winding down and school is back. And all of that has us thinking about this idea that something happens in the human brain when we turn 25. Suddenly, we can rent a car without fees. Make rational decisions. We may even regret some of our past… indiscretions. All because we’re developed…right? Well, a paper in the journal Nature suggests that may not be the case. Neuroscientists found four distinct turning points in brain development across the human lifespan – and that the brain may be in its “adolescent” phase until about the age of 32. (Encore episode)

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This episode was produced by Rachel Carlson. It was edited by Rebecca Ramirez. Tyler Jones checked the facts. The audio engineer was Kwesi Lee.

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