I can't answer the CA law question, but my son had a BB gun at a young age (six, I think), then I got him a .22 rifle when he was maybe 10. Now he's 18 and he has two rifles (.22) and a Hawkins .50 black powder. He has never fired a gun without me being present, but I'd trust him completely if he did.
Since I've always had guns around I trained him from a very young age. Once, when he was 3, he found a flare gun (unloaded) in the attic and ran down and told me about it (never touched it).