Erm... the answers pretty simple. It's on starwars 'behind the magic' cd brought out a few years ago.
Once the empire was formed recruitment began.
Proof?:
*George changed OT boba to kiwi accent, and left the stormtroopers.
*Storm troopers vary in height
*Their voices are different
*The clone troopers were officers, soldiers etc. they made the whole lot.
*The empire formed and palpy didn't need perfect uber obedient soldiers.. especially becuase the kaminoins were burning a hole in the republics pocket at the time. Now officers, troopers etc are all different.
It's all on the 'behind the magic' cd. 😉
George has plot holes, but this wasn't one of them.