Fighting unskilled low level opposition for years and years will cause them to form a lot of bad habits.
When they become adults they'll have accumulated a lot of bad tendencies and patterns they don't realize they have, and need to unlearn then relearn to fight all over again.
Its unlikely they'll live up to the expectations, I tend to think.
They don't have the maturity or intellect to really understand things at that age.
But as he ages, he will fight better compitition continously. When he's 14, he'll fight the best 14 year old boxers in the world. 15, the same. When he's 17, he'll be brought to weight classes or whatever. As long as he's on top and beating everybody, he can't have a downside for the time being.
But I agree, 13 is really early, but that JJ, boxing and kickboxing experince would be insane if he done wreslting in college.