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When can you really consider yourself to have an “adult” brain? People often claim that the brain is fully cooked at the age of 25, making us more rational but also harder to change – but in reality, things are a lot more complex.
Does the brain stop developing at 25?
The short answer: No.
As one 2013 paper notes: The notion that “brain development is not complete until near the age of 25 years refers specifically to the development of the prefrontal cortex.” The prefrontal cortex is part of the frontal lobe, sometimes described as the “rational part” of the brain.
Compared to regions like those involved in learning languages, for example, “The prefrontal cortex, a brain region responsible for cognitive control and inhibition, is slower to develop,” neuroscientist and science communicator Shannon Odell explained in a Ted-Ed video. The area becomes more developed in adolescence, Odell says, and “Adults benefit from a well-developed prefrontal cortex, allowing them to better execute skills that require learning, focus, and memory.”
Multiple sources point toward research from the National Institutes of Health in the early 2000s as suggesting this area matures at the age of 25 specifically. According to a 2005 report by NBC News, the research (which was still ongoing at the time) involved magnetic resonance imaging (MRI) scans of 2,000 people aged between 4 and 26 years old.
Dr Jay Giedd, who led the research, told the Death Penalty Information Center in 2004, “When we started, we thought we’d follow kids until about 18 or 20. If we had to pick a number now, we’d probably go to age 25.”
So will you awaken on your 25th birthday feeling like a butterfly emerging from its chrysalis, suddenly more rational and set in your ways, with a fully-done prefrontal cortex and a fresh perspective on life that was unattainable at a younger age? Probably not.
In that research, the dorsal-lateral prefrontal cortex’s timeline of development was observed to vary greatly between individuals, even being “generally completed a year or two earlier in women.” The reason? "We still don't know,” said Giedd. Also, as NBC News noted at the time: “Critics of brain-imaging research – and Giedd himself – emphasize that there is no proven correlation between brain changes and behavior.”
How does the brain change over the course of your life?
The researchers behind a 2022 paper in the journal Nature made charts of how the human brain grows over a lifetime, looking at the volumes of different tissues in the brain via 123,984 MRI scans from 101,457 participants. The age range of the participants was vast, ranging from 115 days post-conception to 100 years old.
“We generally have an understanding of how big the human brain is, but we have never been able to measure this with such precision or at such a large scale across the whole lifespan,” the study’s co-first authors, Dr Richard Bethlehem of the University of Cambridge and Dr Jakob Seidlitz of the University of Pennsylvania and Children’s Hospital of Philadelphia, told IFLScience.
“There is no single dataset that could answer this question,” they explain. “It has been only through this truly global effort that we have been able to virtually ‘stitch’ together dozens of neuroimaging datasets at a massive scale.”
“What we additionally provide on top of that is a standard blueprint for how the brain typically develops and a common language in terms of converting brain measures into (per)centile scores.”
Not only is there enormous inter-individual variability but also [...] it really depends on the specific property that you are measuring and in which part of the brain you are measuring it
Dr Richard Bethlehem and Dr Jakob Seidlitz
What can this data tell us about how the brain develops in our younger years, and how does this compare to later stages of life? “Perhaps somewhat unsurprisingly we find that most basic morphological properties that we measure have their peak early on in development, many even before the sixth year of life. So changes are much more rapid early on in development, while in later stages of life the changes are more subtle and gradual,” Bethlehem and Seidlitz explained. “We find that the individual variability is highest in adolescence and young adulthood (when individuals with psychiatric disorders are typically diagnosed).”
However, it’s “very unlikely” that there could ever be a one-size-fits-all timeline for brain development over a lifetime, Bethlehem and Seidlitz told IFLScience.
“We find that not only is there enormous inter-individual variability but also that it really depends on the specific property that you are measuring and in which part of the brain you are measuring it,” they explained. “We also acknowledge that even with ~125,000 scans we don’t have a full and comprehensive picture of the true variation in the population as these are still biased towards WEIRD [Western, Educated, Industrialized, Rich, and Democratic] countries where a lot of this research is done.”
So is the brain ever “fully mature”, and how could this affect how we think? “This all depends by and large on how you define maturation. We find that depending on where you look in the brain and which property you measure there are peaks at certain ages where the given property reaches its maximum, but we also see that changes still happen after that time,” said Bethlehem and Seidlitz.
The mid twenties number doesn’t come entirely out of the blue [...] However, this absolutely does not imply that the brain then stops being malleable to change
Dr Richard Bethlehem and Dr Jakob Seidlitz
“There is a lot of research happening (not least in our collaborative network) to see whether some of the developmental milestones we identify intersect with milestones of cognitive development. There is also a large field of research trying to pinpoint specific cognitive domains and functions to specific maturational patterns of different brain systems,” they said. “There is probably an association between how we think and how our brain develops and is organized, but to make studying this practically feasible we tend to break this down into smaller chunks of specific cognitive processes and specific properties of the brain.”
Is the age of 25 specifically significant for the brain? “The mid twenties number doesn’t come entirely out of the blue as it is an age where many different brain regions will have reached their maximum volume for example. However, this absolutely does not imply that the brain then stops being malleable to change nor does it mean that up until that point the brain would not be capable of functioning at a developed level,” Bethlehem and Seidlitz explained.
“The intricacies of how these maturational trajectories of different brain systems and properties intersect with cognitive development are not fully elucidated.”
