Well let's start with theft. Just because the keys are there doesn't mean that's an open invitation to take something.
Then there's the business of driving
- without a licence
- with no insurance
- with no name on the registration.
What do you think? Does that go beyond unruly or mischievous? And these are just general considerations. Wait till insurance gets into it. In come cases they may not pay. So now what?
Four general counts and more to come from the lawyers who deal with this sort of thing all the time. I would say that 14 year old better start worrying a bit.