I started at age 12 using my brothers old POS bench, my son however I started at age ten with things like bench step ups, plyometric box jumps, sprints, elastic resistance bands and pull ups. Soon as he hit the puffy nips of puberty stage we hit the weights... he's 5' 6.5" tall though so I might've eff'd him up...unsure. He's 17 now and benches 280 pounds and squats 405 at 152. His mom is also short (5' 2") and I'm a lowly 5' 9" tall. When I was his age I weighed 210 pounds at 7% bf though. His stature may be more his mother than the weight training. I think that the strength increases in bone density are worth it though as he's a football player and has never suffered an injury, whereas other kids his age he's played with have had plenty of broken arms playing the same position RB. I'd rather have a mentally tough and strong short stature kid than some 6 foot tall kid who whines about poundage's though.