If they have put in the time learning to fly and they are supervised by a knowledgable adult, I don't see a problem with it. My ex's son was flying at that age and now he is a college to become an airline pilot.
Well that would depend on what their parents/guardians thought. There are services to make sure the child gets on the right flight and is taken care of that the parents/guardians can request, but it's ultimately up to them since the child is under their care.
good luck running away at 40000 feet....
With permission and id. Kids do it all the time. Especially if their parents live on opposite sides of the country.
I did at ten.
With parent's permission it's allowed isn't it? My daughter took a flight when she was young.