I fired my first shots at age 7. One of those great memories of childhood. While visiting my grandparents in Indiana during the summer of '71 my grandpa took me over to my uncle's farm with its large open space and taught me how to load and shoot his Nylon '66. We shot cans off fence posts and at ones we tossed in the creek that ran across the property. I had a ball. He taught me to always keep it pointed in a safe direction when not shooting, preferably down at the ground, to always check if it was loaded regardless of how certain I was, and to keep my finger off the trigger until ready to shoot.
You have to know your own kid to know if he's capable of minding the simple rules of gun safety. If you think he's responsible enough to deal with a gun in a real way and not treat it as a toy then IMO that's how old a kid has to be.
I'm good with numbers:
.22 .32 .380 .40 5.56 .223