(As a passenger, not driving alone)

as long as they are old enough to hold on. and wear a helmet.

when they learn to obey and not be a hard head

l would say 8. with a helmet and with and adult only.

in my opion 13 with protective gear Check with your States Department of motor vehicles

l think 10, with a helmet and protective gear. And you should start going slow to make sure they can hold on.