"Parents Supervision" is the point here. My son started at 3, but was riding and doing wheelies on his pushy. He knows the rules for riding the mini bike, and they are:
Yard needs to be totaly clear.
Riding gear, helmet (AS Approved), gloves, jeans, jacket and boots. I don't give a crap how hot it is!
Dad does a test ride first to check that all is working the way it should(Need to have some fun

).
10min revision with dad 2-5m away.
If that is all good he can have fun, and I'm 10m at most from him at all times. The good thing is the concentration on his face is amazing. He understands the hand signals incase he can't hear over the noise.
If he wasn't ready to ride, no bike! as simple as that. He wanted to get a dirt bike but too much Krusty Deamons on TV put an end to that real quick.
As a parent I will take whatever precautions to insure that his riding is as safe as possible, he has allways been around bikes and if learns the correct message on safety, Good.
He will often mention "Look at that stupid in shorts, no gloves ect" He knows what he has to do and how to do it, and I'm never far away!
"I live in my own little world. But it's OK. They know me here"