Just like the human body the human brain changes quickly during puberty then slowly for the rest of your life. There is no magical point of “fully matured”. That’s a garbage paradigm.
Looking for it creates lots of dumb clickbait that people love to parrot tho’
The vast majority of brain development happens during puberty. Nature gives you an adult brain to go along with your adult body simultaneously. Which by the way, is the only system that would make any sense.