Is the Human Brain Fully Developed at Age 25?

The tricky question of when someone’s mind is considered “mature”

Pictured: not fully developed brains?Photo byParker Gibbons on Unsplash

"It’s irresponsible of us to expect teenagers to make decisions. After all, their brains are still developing and won’t be mature until they’re at least age 25!”

I’ve heard this argument applied to everything from signing for student loans, to picking a career, to getting married, to joining the military. This idea of age 25 being ‘the point at which your brain becomes mature’ has saturated the internet.

Where did this number come from? What, exactly, happens at age 25?

And more importantly, is this actually true, or is it a total myth, like the parable of boiling frogs?

Related: The Truth Behind the Myth of Boiling Frogs

Sometimes we’ll put up with anything. Sometimes it takes brain damage.

Spoiler alert: it’s got some grounding in facts, but it’s mostly bullshit.

When does the brain stop changing size?

One of the biggest challenges in saying when a brain “stops” growing is the fact that, in some sense, it never stops. If you learned something today that you didn’t know yesterday, that means your brain created new connections between neurons to retain that information. Our brains are massive maps of information that are constantly updating.

Maybe we can be pedantic and say “the brain stops growing when it stops getting bigger.” That’s pretty easy to measure; we can do MRI scans of the brain and measure its size…

…but even this gets tricky, because the brain grows quickly at first, and then continues slowly expanding after that. Most sites agree that the brain reaches about 90% of its final size by age 5. But after that, some areas continue to grow; the corpus callosum,the bridge that links the two halves of the brain, keeps growing until the mid-20s.

And other areas shift over time. The prefrontal cortex, located (as the name suggests) at the front of our brain behind our forehead, develops more white matter — nerve fibers that transmit information between cells — throughout our teenage years and into our 20s…

…but also beyond that. In fact, the brain’s rebalancing continued all the way to age 30, the oldest data point in the above 2016 paper in Neuron. It’s highly likely that it continues changing for as long as we’re alive.

So, 25 isn’t when the brain stops physically aging. Maybe it’s about activity patterns?

Thinking like a teen

Teenagers are known for being moreprone to make rash, impulsive decisions. And teenage years mark a heady time for brain development; hormones canexert strong influences on behavior, and many teenagers carry the confidence of an adult without the experience and wisdom that comes with age.

But does this show up in the brain?

Some studies have looked at brain performance and found that, when placed in stressful conditions, teenagers perform worse at certain tasks like identifying emotions in faces. People do seem to get better at this task with age, but again, there’s no direct cutoff at age 25. People aged 18–21 performed worse than people who were over 21.

Another wrinkle in judging brain performance is that these supposedly “adult” traits — maturity, wisdom, ability to perform in stressful situations— vary widely from person to person. (Have you ever met a young child who seemed “very mature for their age”? Or an adult well into their 30s or 40s who still “needs to grow up already”?)

This high degree of variance, which also shows up when trying to obtain more objective data-driven results like functional MRI scanning of brain activity, makes it very challenging to try and identify a brain’s age based on its activity patterns.

So what does happen at age 25?

Perhaps the idea of a “mature” brain not happening until age 25 is based more around life events and the structure of our society, and less around science.

For many middle-class and upper-middle-class young adults, age 25 is when a long-term plan may finally settle in. This is usually the point where college students have graduated and settled into their first job, if they were fortunate enough to land one.

In the past, age 25 was close to the average age of getting married, but that’s now closer to age 30 (30.5 for men, 28.4 for women).

It’s approaching the average age of having a first child: 27.3 years old for a first-time mother.

In the past, this may have been when people started considering settling down, although the average age of a first-time homebuyer has increased to age 36, as of 2023.

Perhaps the idea of age 25 being the age of adulthood is less about the brain, and more about the time it takes in modern society to achieve the milestones commonly associated with adulthood. If that’s the case, we may see the statistic continue to climb.

Will age 30 be the new “I’m finally an adult” mark?

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One of the big challenges with science is that it’s rarely possible to distill a complex topic, such as neuroscience, down to a simple factoid. Our brains constantly change and grow over time, and the timeline is different for each individual. There is no easily measurable threshold that indicates when someone is a “real adult,” or is fully mature. Maturation is a spectrum, not a binary.

Perhaps a better philosophy is to remember that our growth is not done by age 20. Even after we are no longer teenagers, we can continue to grow and learn — and we’re still at risk of making stupid mistakes, especially if we’re under stress.

There’s nothing magical about age 25. Each year, as you add another candle to your birthday cake, feel good that your brain has continued to grow and change!

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