[zenithd]

> and not being able to get much stronger with training

Actually, the problem is the opposite. Their muscles get too strong too quickly and then they blow a pulley because their tendons aren't strong enough for the forces that their muscles are easily capable of producing. This happens in adults too, but AFAICT it's more common in kids and adolescents (but perhaps that's because most adults who get into the sport later don't ever climb as hard). Growth plate injuries are less common but devastating.

Avoiding these sorts of injuries is probably the biggest reason why training or a team is worth the $.