You're allowed to legally disown your family at 15, but you have to go through court and prove you are able to live alone (school, job, car/ride, house, etc.) but if you don't do that your parents are still legally responsible for you, so no.
Yes and No. On one hand I think yes because in case of abuse for example I think it is better for the kid to move out than to go to foster homes. But only if they are 16 or more because living on you're own requires a big responsibility and I don't think that kids younger that 16 should be exposed to that kind of decision.