Legal Guardianship: How would I find out who my legal guardians are, and can I appoint one?

Long story short, I've had a very turbulent life, and I've lived with several family members, including my mother, father, and grandparents on both sides. I need to know if my grandparents on my father's side can be considered my legal guardians, considering I've been living with them for about two years, and about five or six before I moved back in.

I'm currently 20, and I still need to list legal guardians on some paperwork, especially involving taxes. Is there a place I can call to find out who my legal guardians are? At 20, is it too late to have my grandparents on my father's side to be considered legal guardians, if they're not already?

Thank you for your answers!