My seven year old daughter likes to help. The boy was pretty useful at around 8, and much more so now (at 14)--he can stack and enjoys moving the big ones. I split a pile w/out his help this fall and I got less than half as much done.
Make sure everybody wears hearing protection as mentioned, and keep a good eye on the kid. All kids aren"t the same at that age, and you know your kid better than we do, so you should make the call (yes, I would trust my 7 year old more than my 14 year old in many situations--the boys mind tends to drift and he seems to think he"s bulletproof).