I started shooting with my dad at about 5 or 6 years old. I was tagging along on hunts by 6 or 7. I was actually partaking in the hunting by 9. By 12 I was shooting trap at the local gun club... That was all in the mid 80's though, and times are much different now. My folks didn't have to compete with MTV and Liberal Activists back then.
I think you, as the parent, have the best judgement of your kid's ability to control him/her-self. If you think the kid is ready, then get him started. If you have apprehensions about his maturity (which is completely understandable at his age), then I suggest breaking him in at a later stage. I never suggest toy guns (BB or airsoft) for education. I think you lose the threat of "DESTROYED FOREVER" with a toy. Shake a soda can, set it up about 15 yards away, and shoot it with something big. The end result is impressive to watch, and the damage should leave a lasting impression. It's a very descriptive picture of DESTROYED FOREVER.
I personally can't wait to get my son started on shooting (he's 5 months old, so I have some time). I fully plan on having him competing pre-teen competitions. I come from several generations of hunters/shooters. My wife shoots alongside me and encourages teaching our son as early as we can.