I think whenever you can get a full-face road helmet (not a dirt bike one as IMO they are not really suitable for the road) to fit, and he can reach the footpegs.

I've been riding for 10 years now, and the bikes are just getting so powerful... just a small twist of the throttle and it may well be too much for a littlie to hang on tight enough.

But I'm all for starting littlies on PeeWees and the like in a safe off-road environment! All the riders of the future must start somewhere.

Four wheelers, from my experience, are even more dangerous - it is too easy to get trapped under them if they flip/tip.