There is no data to show that hard exercise damages growth centers in bones. There is no evidence that growing larger muscles stunts growth or interferes with coordination. In the May issue of the journal from the American College of Sports Medicine is an article showing that just 4 weeks of hard exercise in growing animals increases bone mass. That means that children who start training while they are still growing will have a great advantage over athletes who start training after puberty. In most sports the strongest athlete wins.
Muscles can only grow to be as strong as the strength of the bones on which they attach.
People with the biggest bones are the ones who can grow the biggest muscles. As far as their bodies are concerned, young children can start training at a very young age for athletic competition, but as far as their minds are concerned, they should not train before they are ready to accept the regimented lifestyle required for athletic competition. There is great concern that some children will be subjected to abusive coaches and inconsiderate parents who place athletic training above the child's own wishes and desires. The concern about serious athletic training for young children is more mental than physical. Children should not begin serious athletic training unless they want to do it, that they take days off from training regularly and when they want to, and that their coaches and parents allow them to be children.