Adulthood

I turned 20 years old a few months ago. I am no longer a "teenager" but at the same time I don't feel as though I am an adult. When do you become an adult?
Is it when you reach a certian age, like 16, 18, or 20?
Is it when you get a job and can support yourself financially?
Is it when you move out of your parents' house and begin living on your own?
Is it when you get married or have your own family?

Sometimes I have to stop and remind myself that I am not 14 years old anymore. I am an adult even though I may not feel like I am.

To me being an adult means more then not being a child anymore, I just can't figure out what exactly it is that it means.