Hi Michael. Studies have shown that strength training can be beneficial at very young ages as long as form is correct. There are many people that think lifting weights will stunt growth, but that is actually false. Weight lifting and exercise actually promotes growth because it stimulates the body to release growth hormone. Therefore the only constraint to strength training would be motor control and development. Adolescents develop at different rates, but I would say 10 probably wouldn’t be a bad age to start.