My sons were 6 and 8 when they learned to fly. They both soloed and then didn't want to fly anymore. There is only one piece of advice that I will give here, and I stand behind it 100%. Teach your kid ONLY if they want to do it. Do not force them, and try to make sure that they're not doing it because it's what they think you want them to do. Too many parents these days push their kids into sports or activities that the parents think the kids should be in, but the kid doesn't have any interest in. All this does is breed a hatred for that activity in the kids mind.

If the kid wants to do fly, then there is no age too young. I have see a 5 year old doing full out 3D flying better than most adults can do.

Hope this helps

Ken