Underage kids driving motorcycles isn't the real issue, Not out in the sticks I see it in the villages every day kids as young as 6 or 7 riding to school on motorcycles, I know its not ideal but the ones I see ride reasonably slowly and carefully, That all changes when they reach adolescence, and that can last for 30 years or more in Thailand
The reason the police don't bother the kids, is that the age of criminal responsibility is 15, no point looking at the parents as they will just say the kid stole the bike, and the kid will agree.