As neon said it depends on where you live. Try looking it up on the Internet. Check the family code for your state or just type emancipation in whatever state and it gives guidelines.
Here in Texas there's a fine line when petitioning to be emancipated. If I parent requests to have a child emancipated due to various reasons, it could become a case for children protective services.
Note to Ema: asking why is a valid question since the topic was started saying wanting to get separated from parents. Some people so simple minded. One dimensional brains