There are attributes that don't change between 12 and 21. One of those is brains. You're either born with them, or you're not. Problem is when the system is setup in such a way that it values the most, during the developmental years, exactly those traits that can be misjudged in teens: strength, size, speed. A boy hitting puberty at 12 will be the biggest, fastest, strongest at 15. But, then, the late-bloomers reach and surpass him at around 20. But, if by then the system has pushed them aside, well, it's too late: they are stuck with a bunch of 21 year olds who are no longer any bigger, any stronger, any faster than their mates, and are completely mediocre in all other aspects.