Ok I don't want to step on anyones toes here, but...
the skeletons of medieval archers very often show that they were deformed, having one shoulder cradle a different size from the other. This did not happen when they were adult. It happened when they were children because the law at the time demanded that they learned to shoot a bow.
A young male does not stop growing until they are around 21 years old. During that time there is a soft part of each bone, just off the joint, where the growth happens. If you give a youngster a heavy bow, and he or she shoots it on a regular basis, it can actually stop the bone from growing, hence the lop sided shoulder growth.
It doesn't matter if they are huge farmers boys at the age of 13 or 14. They are still forming their skeletal structure so damage can be done. Keep the weight down and make sure their form is paramount.