Been thinking about this a little more. I did not come from a family that hunted or used guns. There were a few around but they were never shot. As a result, I started extremely late, which was only a hinderance. I agree with those that are saying maturity should dictate the starting age.
I think a kid should be started as early as their maturity level will allow for safety. The longer you shout the more your skills will develop.
My nephew is finally mature enough that I think he would be ok. He's 11 now and up until a year ago I'd have said no. Unfortunately his parents are afraid of guns, which means he'll probably never get the chance to shoot with his uncle until he's an adult. But I'm not likely to have kids at this point, so I'll take it.
My niece in the other hand, I wouldn't trust with a butter knife.