I didn't read all of the responses, so this may have been said already.
If a person is:
haven't been married
don't have any children
not a veteran
hasn't been a foster child/adopted/ward of the court
They are considered to be "dependent" according to the guideline of the Dept of Ed., regardless if they moved out at 16 and totally support themselves.
Buuuut, just because a parent fills out and signs a "Parent Plus" application, doesn't mean they'll be co-signing on any loans. At a career/trade school 90% of parents are declined and even if they are approved, just filling out the app and running it doesn't mean they're on any loans.
Regardless if they're approved or not, a dependent student must provide the parent's tax returns to be used for the FAFSA app.
I speak to a lot of parents who refuse to "be on" their child's FA. I explain to them to let us at least check, it's a good chance they will be declined. They usually are, and all of the loans are in the student's name, but we have to have the parent's income tax return to complete the FAFSA and have them sign the Parent Plus form to see.
Not the school's rules, that's a Dept of Ed rule.
Every now and then you can get a Professional Judgment, if the parent is in or has been incarcerated and there is proof that the parent is MIA.