I feel like as soon as they can under stand stop, go, turn, slow down, and go faster they are old enough to ride. I bought my 2 year old daughter a JR 50 and made training wheels for it. She still doesn't get the slow down thing slow I haven't let her ride it yet. I would say as soon as it get warm out she will go on her first ride.
Really she was 1 1/2 in those pics