My boy started handling them at age 4, looking through the sites and memorizing (and following) the 4 rules, as well as eddie eagle's rules.
At 5, he was shooting at the range with my assistence (Ruger 77/22 was too big for him to hold).
At age 6, he was shooting alongside me, and shot his first handgun. I still don't have a gun that fits him, I have to get him a cricket one of these days.
But he's an interested, reasonably grounded kid.
My girl, now age 4, is a big ball of silly right now and can't concentrate on mych so she'll have to wait a bit longer it seems.