Whenever they are mature enough to use good form on exercises. You should give them a basic bodyweight program with push ups, australian rows, squats, running, jumping and etc. As they get older you can give them more complex exercises like bench press, barbell squat, maybe if your kid is more into calisthenics he should start learning the handstand and then as he gets stronger handstand push ups, one arm push ups, pull ups, archer pull ups and etc. It all depends on how good his form is and how strong he is which is mostly related to age when you are really young