Is there suck thing as power of attorney over a child when there 19 years old?

My girlfriend has a bad situation with her mom at home and her mom said that she couldn't do what she wanted or move out because she has power of attorney over her medical stuff and she is 19 years old and her mom said your age doesn't matter. Is that true or is her mom just telling lies because i have never herd of something like that were if you have control over there medical stuff you have control over them.