People argue this point all the time, but the answer depends on the child and when their athleticism manifests itself. Obviously, children are continually developing their muscles and continually learning to control and use those muscles, so once a child has developed fundamental athleticism, why not implement weight routines to further strengthen and develop those muscles, especially with complex movements which integrate multiple muscle groups. This would not only enhance muscle control and further coordination and balance – it would also add strength, size and performance – just like it would an adult. Muscles are muscles. If your child looks athletic on the field – whether he’s 9 or 13, he’s probably capable of conducting complex resistance movements which will contribute to his overall development.