In some cultures/religions adulthood begins at age 13, which is the age when puberty usually starts obviously. The kid has been trained to tackle and defeat the opponent, that is what he did. Obviously what he did was wrong, but he did what he thought was right at the time.
A discussion no football fan wants to discuss is, the harmful effects of playing football. Endless injuries including both mental and physical. I could see a lawyer possibly defending the kid (guy hasn't even left high school at this point) in court, if it went that far, that he was a victim of an aggressive sport, that went to far. In the heat of the moment sort of thing.
I also heard that mental development isn't reached until age 25. In any event, I think the kid should do community service, see a therapist, and be banned from competitive sports until he is cleared by a medical professional. Jail time seems a bit over top in my opinion, but an apology is in order for sure.