As with all population-wide things, it’s a range: some bodies mature sooner, some later in life. The number 25 is a convenient mid-range estimate.

Let’s take an example that we can actually see, and use it as a rough marker for physical development: wisdom teeth. The majority of people get them between 18-26. In some instances, however, the wisdom teeth don‘t erupt until the age of 40! This indicates that the individual’s body is still developing at that age.

Hope this makes sense.

The brain changes all your life. That you call it development or degradation depends on your interpretation.

Impulse control improves until 20 years old and is pretty much the thing that improves the latest.

The ability to learn many things including languages peak much younger and is already significantly degraded at 20. So it's not a continuous improvement.

From a general science perspective (I have no specialist human-body expertise) I would suggest that it's arguable that development is asymptotic - the rate-of-development or mental-maturing slows down as you approach the end-point, but never quite gets there.

I would interpret any figure (e.g. age 25) for being "fully developed" as an indicative ballpark rather than a discrete finishing line that you cross.

My understanding (non-expert) is that the brain is constantly developing through life, making new connections, letting old ones weaken...

The general public, the popular press, and the legal-folks crave certainty and absolutes. In real life, things are generally a bit fuzzier!

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In physics and electronics we often measure or define when a switching signal gets to 90% or 95% of its final value, as defining when you reach 100% is really difficult (and not practically useful).

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