I figure right after they hit their growth spurt, unfortunately mine was up to 5'7-8" . Try aroung 15 and make sure the doctors tell you how much growing he has left since they will be able to monitor the growth plates. They might have stunted your growth i couldn't tell you but make sure your kid squats very lightly untill he's done growing since the back is very fragile at the younger age, and I think I remember my professor saying the legs finish growing the last..... so leg excercise should be a minimum let him do sprints.