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).