When is your brain actually an "adult"?

There’s 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 recent paper in the journal Nature https://www.nature.com/articles/s41467-025-65974-8 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. Interested in more of the science inside your brain? Email us your question at [email protected] mailto:[email protected]. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. This episode was produced by Rachel Carlson. It was edited by Rebecca Ramirez. Tyler Jones checked the facts. The audio engineer was Kwesi Lee. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:18Regina Barber?

  You're listening to Short Wave from NPR.

• 0:24Regina Barber?

  There's this idea that something happens in the human brain when we turn 25. Suddenly, we're developed. We can finally make rational decisions. And we might even regret some of our past indiscretions.

• 0:37Speaker 02

  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.

• 0:54Regina Barber?

  Duncan Astle is a neuroscientist and professor at the University of Cambridge. 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.

• 1:09Speaker 02

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

• 1:12Regina Barber?

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

• 1:16Speaker 02

  And then it gradually starts thinning.

• 1:20Regina 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?

• 1:30Speaker 02

  Well, it was seen kind of as this sort of unfathomable question. You know, it's this question that neuroscientists hate to be asked.

• 1:37Regina Barber?

  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.

• 1:49Speaker 02

  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.

• 2:00Regina 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 Short Wave, the science podcast from NPR.

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• 3:54Regina Barber?

  OK, 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?

• 4:11Speaker 02

  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 it'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.

• 4:35Speaker 02

  And then from nine to thirty two, what the, what the brain is doing is wiring those up. And so you, so you start to get strong myelination of long range connectivity, which connects those clusters up together. And so in that long nine to thirty two phase, what the brain is, 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

• 5:08Regina Barber?

  Like efficiency.

• 5:09Speaker 02

  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 thirty two. 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, you get really a, a much more stable like pattern.

• 5:29Regina Barber?

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

• 5:52Speaker 02

  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.

• 6:03Regina Barber?

  Hmm.

• 6:03Speaker 02

  Um, when they're designed. So yeah, up to sixty six, it is really very stable. And if you look in the data really carefully, you can see that that turning point is sixty six. It's much more gradual and subtle

• 6:15Regina Barber?

  Hmm.

• 6:16Speaker 02

  than the other turning points. And to, 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.

• 6:30Regina Barber?

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

• 6:39Speaker 02

  Exactly. And I, and, and it's a 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 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 ninety percent of the time, it it will still work pretty well. It's a bit like like an orchestra that's learnt a piece of music.

• 7:23Speaker 02

  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 eighty three onwards, you see that certain key hubs start to play a really crucial role.

• 7:44Regina Barber?

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

• 7:51Speaker 02

  Yeah, that's a great question. So

• 7:56Speaker 02

  in a way, I guess what we're looking at is a sort of average brain for each age. So we've got the four thousand 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 like an average brain for each year of life. And you can imagine that you're smoothing over an awful lot of differences between individuals that you're kind of losing in the days. Now, we know from other studies that things like cardiovascular health and social connectedness are really important for maintaining brain health into older age. And also, we're pulling out one particular aspect of the brain, this idea of the kind of insulation of the wires and and using that to look at organization.

• 8:39Speaker 02

  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 that we're not able to look at through this kind of one particular lens that we've chosen.

• 8:59Regina Barber?

  Yeah. So this adolescent brain, if you think about it, you know, nine year old brain is not the same as a thirty two year old brain like. Just in the way we live, why make that one category?

• 9:14Speaker 02

  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 the point is that they are there and so 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.

• 9:56Regina Barber?

  Give me some examples. What do you mean?

• 9:58Speaker 02

  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 that because of course we're 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.

• 10:40Regina Barber?

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

• 10:47Speaker 02

  So I think one thing is interesting that has struck us is that

• 10:52Speaker 02

  different you will encounter different types of conditions in these different phases of life. So for example, if you are gonna if you are gonna 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 gonna have a mental health condition, your anxieties, depressions,

• 11:15Speaker 02

  also kind of more severe mental illness like schizophrenia.

• 11:20Speaker 02

  Adolescence is when you will first have the onset of symptoms. So seventy five percent of all of those conditions will have their first onset before become twenty four or twenty five. And then, of course, when you think of the other end of life, you can think of kind of dementia, age-related cognitive difficulties, Parkinson's and so on.

• 11:40Regina Barber?

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

• 11:48Speaker 02

  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 twenties, early thirties? We haven't traditionally thought of that as a particularly interesting developmental window, because we've assumed, you know, you're eighteen, sorry, you're an adult that's sort of eighteen or twenty one or whatever the law says.

• 12:15Speaker 02

  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 lines or whatever.

• 12:32Speaker 02

  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.

• 13:16Speaker 02

  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 have 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.

• 13:47Regina Barber?

  Wow.

• 13:48Speaker 02

  And so I actually think this idea of, you know, it takes this tool with thirty to develop. In a way, I sort of feel like this is kind of our secret weapon as a species relative to other species.

• 13:57Regina Barber?

  That's fascinating. I had never thought of that.

• 13:59Speaker 02

  Personally, I think, I mean, I became a parent about eighteen 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.

• 14:18Regina Barber?

  Yeah.

• 14:18Speaker 02

  I'm like, guys, we've got like another like thirty point five years left to go. Let's, you know, chill out. We can't, we can't keep doing this. And so I now feel incredibly relaxed.

• 14:29Regina Barber?

  Yeah.

• 14:30Speaker 02

  At the idea that development's really long. People are on really time, different time scales. Early and fast is not always good.

• 14:37Regina Barber?

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

• 14:43Speaker 02

  My pleasure. Thank you very much for having me.

• 14:51Regina Barber?

  If you liked this episode, follow us on the NPR app or whatever podcasting platform you're listening 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 I'm Regina Barber. Thank you for listening to Short Wave from NPR.

• 15:36Regina Barber?

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