My oldest is 7 and I let her shoot a pump pellet gun.. It will be awhile before I let her move up from that.. Very small she is.. Its up to the kid and the parent.. Some kids are ready faster than others.. Its a judgement call..
I probably got my first BB gun when I was 6 or 7. I remember I had to cock it by bracing it against my foot, holding the barrel with 1 hand and pullin the lever with the other. Man I musta been a weinie little brat.