I started teaching basic gun safety to my son when he was 6... Untill I am sure he's got it 100% that's as far as we go. Right now be handles my paintball gun.
(He's 7 now). Keeping it simple for now:
1. Only point and what you want to kill
2. Finger on the trigger only when you are ready to shoot
3. When not doing the above, gun is pointed at ground or sky.
Really depends on their maturity level. I know my son is not quite focused enough to have him controlling one independantly. WHen he is focused, no issue, but *SQUIRREL!!* sometimes he gets distracted.