Spilled Expresso

When does someone become "grown-up"?

I celebrated my 23rd birthday a few weeks ago. Like prior birthdays of the past five or so years, the day caused me to reflect back on my life and the prior year. In all honesty, the time between age 22 and 23 was rather uneventful and unproductive, and will most likely become known as the "forgotten year."
I guess I didn't know what to expect from my early twenties. As a teenager/high school student, people in their twenties seemed so grown. At 16, I would have confidently assured that a woman of 23 was an adult, but now I'm not sure. What defines an adult?
Along the way, there are many so-called markers of adult hood. I didn't feel like an adult after turning 18, I was still in high school for a month and living in my mother's house. Nor did I feel like an adult at 21, still residing in college dorms. Even now, living on my own, with college degree, I still don't feel like an adult and I'm not sure when I will.
As I look a round at other people in my age bracket, I wonder when they became so adult. Every time I go on facebook, I read another status message about someone my age getting engaged, getting married or pregnant, buying a house. I wonder if they feel like adults.
I know that getting engaged isn't the turning point of adulthood, or else the country would be full of children pushing forty. Is being an adult a state of mind--like happiness? Do you recognize adulthood when you reach it, or is it something you recognize only on nostalgic reflection. Any twenty-somethings reading this-feel free to respond with your thoughts on if you feel grown and so when