It is fine if certain precautions are made. I've known two teenagers who have motorcycles. One has had many accidents including the skin of his hands, arms and legs getting rubbed off horribly raw and bloody, yet he still cared more about the bike than himself. The other is more cautious and has never been in an accident, because he values himself over the bike. It depends on how safe the individual teen is. How much testosterone do they have? How proud are they? Do they give into peer pressure easily? All of the answers to these questions should be no or low for the teenager to have a motorcycle and be responsible with it.