As a baseball dad and a lower level 30+ year Olympic development coach I would just like to add to what 22and25 said above. Pitching stresses the body and the body needs time to recover. If the body does not recover you are breaking it down, not building it up. Some pitching stresses the body more than others (for instance a 95 mph fastball stresses the body more than a 85 mph fastball). Given time the body will adapt to the stress (increased musculature and connective tissue). This adaptation will plateau unless the stress is varied (weighted balls, long toss, etc.). As a coach I try to keep a long term development (LTD) view and gradually increase workload and give time for optimal rest. The reality is that every kid is different, few programs in any sport keep track of athletes very closely, and eventually it come down to the athlete knowing their own body.