My son was crazy about bikes when he was little. For his 5th birthday, we bought him a little 50cc dirtbike. Of course, with him it was full throttle or not at all, so it didn't take long before he had laid it down a couple of times....not fast enough to get hurt, but enough to take the gleam out of his eyes a little. He's ridden trail bikes since then but nothing on the road.
They can be so much fun and much more free than a car, but you can't trust the people around you and there is virtually no protection. Too bad.