Oh, bull-chit. The brain matures at 25, and until sometime in the 20th Century, we were damn well expected to be adults by age 21.
I think the founding Fathers were pretty close.
Most of us don’t grow up until about 32.
You wake up, look in the mirror and see your Father.
You can do great things prior to that, with guidance and discipline, but one day you get the self wake up call.