What defines an adult?

We spent quite a bit of time discussing this question in class and I have to say that for me, the answer is completely personal. There is never going to be an over-arching definition of terms like this because this is not an objective term -we couldn't even get a class of less than 70 people to agree!
Being called an adult is much more of a psychological state in my mind. I study Film and Television at the Tisch School of the Arts and I characterize myself by telling friends I will never be an adult because it is only a state of mind. I feel like Peter Pan in the sense of not wanting to grow up...what's the point? I am fortunate enough to have chosen to 'play' for the rest of my life by making movies.
Ok, I guess I will be an adult -you could say I am already in a sense- because I have certain responsibilities and have deep down always considered myself more mentally mature than most of my peers. But I stick to the fact: Only I can decide when I'm an adult because no one knows me better than me; and that goes for everyone.