Parents and doctors often say it's unsafe for children to train with weights until after puberty. The thinking is injuries can stint their growth. Why are we letting our pre-teens get beat up playing football or hockey but we don't let them in the gym until they're 14? If they're playing sports then they can lift weights. This report dispelled the myth back in 2001. It basically said adolescents (and even pre-adolescents) could safely strength train to maximize their performance and rehabilitate their injuries.