I have seen childrens Learn to Knit kits at the hobby stores with large plastic needles and they are recommended for ages 6 and up. I'm not sure you can stamp and age on it. I would think that it depends on the child and their ability to pay attention and will to sit still. I don't think it would be worth the aggravation to try to teach a kid whose attention is all over the map or a kid who prefers to play outside until they are around 12 or 13 years of age.
Having taught kids for years I can honestly say, it really just depends on the kid and if they really want to learn.
Knitting to keep my sanity. It's working... kinda...
OTN: Dr Watson's Cable Crew Neck
OTN: Writer's Cardigan