You also need a driver's licence to ride one, so you can't go giving them to five year olds. This also means that if you are caught breaking the law (riding at night, riding unregistered, etc), then you lose you driver's licence.
Want a better idea? Throw a 33cc chainsaw motor on to a push bike and pull 90km/h out of it without the need for registration, insurance, a proper helmet, etc.