When my son was 9 he and I were at the range and he was shooting an AR15. A fella I didn't know came up and told me kids should start out on a 22. I replied that my son did, but that was 2 years ago. He left it at that.
When I'm asked what the minimum age is to participate in one of my shoots I tell people that parents are the best judge of that and so we don't technically have a minimum.
JoeZ