Adolescence lasts into your 30s, and other surprises about the brain

Maggie Penman

The human brain has four distinct turning points where its structure changes, according to a study published in the journal Nature Communications, demonstrating that brain development is not as linear as you might think.

“It’s easy to fall into this belief that there’s a ‘good’ or ‘bad’ way for a brain to be structured,” said lead study author Alexa Mousley at the University of Cambridge. “And that’s not really the case. What this study is emphasizing is the brain is expected to be doing something different at different ages.”

In the new study, Mousley and colleagues looked at around 4,000 scans from healthy people ages 0 to 90 and analyzed their brains. They found four major times when the brain underwent developmental changes, around ages nine, 32, 66 and 83, dividing the lifespan into five distinct phases.

“It’s yet another very nice example of how the brain and its global interactions change across the lifespan,” said Seth Grant, a neuroscientist at The University of Edinburgh who wasn’t involved in the new research. “The message is, there is continuous change from birth until old age. It’s not as though you suddenly build a brain and it stays the same and then just drops off at old age. It’s always changing.”

Mousley and her co-authors identified five epochs during which the brain is wired in different ways.

Childhood

From infancy to nine years old, the brain is busy. There is a lot of consolidation of neural connections happening, competitive elimination of synapses and rapid increases in gray and white matter. But interestingly the brain is becoming less efficient during this time - so it takes longer for information to get from one region of the brain to another. The researchers don’t fully understand why this would be the pattern, but they have some theories.

“We know that in very early life, the brain makes more connections than it needs, and then it prunes them away,” Mousley said. “It’s unclear if that is kind of what’s happening here, but it is potentially what’s happening.”

Whatever the reason for the brain becoming less efficient during childhood, it is a time when a lot of learning happens - language, motor skills, speech - and there is likely a reason that the brain is structured the way that it is during this period.

“It could be that this decreasing efficiency is potentially related to this incredible moment of learning,” Mousley said.

Adolescence

There is a dramatic turning point that the researchers saw occurring around the age of nine on average - a time when many children begin to enter puberty. The brain switches gears and starts rewiring to become more efficient.

The adolescence phase the researchers identified lasts for two decades, into the early 30s on average. This is when people are most vulnerable to developing a mental health disorder, but it’s also a critical time for brain development.

“It is really important to think about adolescence as this protracted window,” said Katie Insel, a psychologist at Northwestern University who studies how the brain changes over the course of adolescence. She said that while in our society we may think of 18- or 21-year-olds as adults, this research adds to a growing body of work suggesting that the brain isn’t fully developed or stable until our late 20s or even early 30s.

“Something that sets us apart as humans from other animals is how slowly we develop,” Mousley said. “A giraffe can stand up very soon after being born, but human babies just take a very long time to learn to walk, to eat.”

Mousley suggested that this slower development might give humans the opportunity to develop more complex brain connections, and could be related to the things that humans can do that other animals can’t.

Adulthood

Adulthood is the longest phase - lasting for more than three decades from around 32 years old until around 66 years old.

“It does seem to be this kind of period of relative stability,” Mousley said. “It’s consistent for a very long period of time.”

That doesn’t mean that the brain isn’t changing during this period, but the changes are less dramatic than during other phases. This is also a period of stability in terms of intelligence, behavior and personality.

“If you just think about what an adult is compared to a teenager, you kind of assume there’s kind of a level of stability there in terms of how people are behaving. And that’s aligning with this three-decade period of consistent brain rewiring from our study,” Mousley said.

Early aging

Around 66 years old on average, the researchers saw another turning point. This is a time when the brain seems to become more vulnerable to age-related diseases - but the news isn’t all bad for the aging brain.

“There’s an expected and healthy, typical way for the brain to shift,” Mousley said.

Insel noted that in addition to some of the negative changes people might associate with aging, like memory loss, there are also positive changes. Older adults tend to be wiser and better at emotional regulation.

“There are pros and cons to every developmental stage,” Insel said. “I think with every phase of life, there are trade-offs where some types of cognition and behavior are privileged because of how the brain is responding to the environment.”

Late aging

From 83 onward, the researchers identified a “late aging” phase.

“What we’re seeing during that late aging phase is something called ‘increasing centrality,’” Mousley said. Particular regions of the brain become more important than others during this time. There is reduced connectivity, but there seems to be a pattern to that change.

The metaphor Mousley used was that of changing bus routes. If you had a direct bus to work, but one day it stopped running and you had to take two buses, the transfer station would suddenly become very important. She theorized that the brain might be prioritizing important connections if other connections drop off.

What it means

The word “development” is often associated with childhood or the teenage years - but what this new research demonstrates is that the brain develops continually throughout our lives.

“We often ascribe certain brain changes to negative outcomes in adulthood or later life,” Insel said. “But actually there’s certain cognitive features that can be really helpful and useful in aging.” By zooming out and looking at how the brain changes over the course of our lifetimes, Insel hopes that we can have a better understanding of what to expect at these different ages, and why our brains might be more vulnerable to certain disorders in adolescence or older age respectively.

Yaakov Stern, a neuroscientist at Columbia University, noted that a good next step would be to try to understand exactly how these measures of the brain might be related to cognitive processes - essentially connecting the dots between this research and other work that has looked at the way our brains function throughout our lives.

He added that many of the things that affect brain development are within our control - such as diet, exercise and social connection.

“The brain changes with aging. We know that,” Stern said. “What interests me, is there are exposures that seem to be associated with more successful aging.”