As long as you don't let him ride on an open road (aka only in your yard or in a field with no one else around) then it's cool..
Anyone who let's someone that's under the legal age knowingly ride on an open road with the chance that they can injure others should be smacked upside the head. I don't care how mature, they SEEM, they still aren't truly aware of how dangerous a vehicle that can move more than 15 miles and hour, can actually be. It's a recipe for disaster. My 13 yr old rides an electric bike that goes 15 mph and has no clue how dangerous it is because to him a bicycle, in ANY form, is more of a toy than anything else.. Same thing with paintball.. To him it's kind of a toy, but I explained to him how dangerous something like that is, by letting him get hit by ONE paintball from 100' away... After that. He got the point. Unfortunately with a motored bike, that lesson can't be learned without major hurt.