I'm 18 and can't stand up to my parents... help!?

First of all legally you are no longer their "property" you are an adult. And i wont tell you when maturity begins it starts at all ages. I dont know you :p. But anyways you have to explain to them you are no longer a child. I think what they want to see you do is to stand up for yourself, then they will know that they have to let you go because as i said before you are an ADULT. Talk to them as you would talk to any other adult. :)