The story starts with his being drafted INTO service, firstly as an AT-AT driver, where he basically picks up on the one weakness that At-At's have & for his troubles he is carted off into the service as an Imperial Stormtrooper. He IS NOT A CLONE! lol
Anyway don't want to start a HUGE argument so how about we agree that there is a mix of Clones & non-Clones in the Stormtrooper army by the time the OT occurs? That way noone has to admit they were wrong & we can all get on again & enjoy a nice hot piece of pie with some hot chocolate whilst we smoke long Cuban cigars :>