TO be honest age is poor indicator of when kids should start shooting. I Know adults well past 50 who shouldn't be handling guns, and I know 5 year olds who can handle them safely. Its more to do with how you teach and how they respond. If they are safe, and understand, any age will do. They must be able to respond to your commands, and do it quickly.
I stay away from bb guns, as the steel shot tends to bounce around, and ricochet is a problem, pellet guns are much safer in that regard as the lead does not bounce nearly as much as the round steel bb's.
Start em early and have fun. Its one of those skills grandpa's should be teaching their descendants. Its a wonderful way to spend time together, and it creates memories that last a life time.
Fwiw, I start all the kids I teach with a small 22 and I use a red dot scope. It makes learning much more fun, and they learn quicker as all they have to do is put the dot on the target. It helps to keep the eyes open, and it helps them focus on the fundamentals of shooting. You can take it off later, and teach them how to shoot with open sights. They become expert shooters from start to finish much more quickly when you use a red dot start in the beginning. Have fun