Young doesn't necessarily mean stupid. Teens lack the ability to understand consequences of their actions which in a competitive environment can definitely be an disadvantage.
Teens often are prone to taking risks which is probably the most determining factor in a game like csgo. So many rounds lost when people challenge when it's completely unnecessary. Obviously there are exceptions in this and all other cases.
There's much more to the subject than that, people are very different at various stages of their puberty and life in general.