The school of thought is that your body will use up valuable energy and growth hormone that should be going toward growth spurts, and instead waste it on building muscle and bone density. I have no idea how accurate this is, but most bodybuilders start lifting around fourteen. BTW I have three requests for you:
1. Get a personal trainer and work out a balanced schedule.
2.Don't over do it. Don't try to lift like a big boy.
3.DONT USE STEROIDS UNTIL YOU ARE AT LEAST 21!!! YOU WILL STUNT YOUR GROWTH PERMANENTLY!!!