When did you first begin to feel like a "grown-up"?

I turned 24 this past July and I don’t feel like what I imagine it should feel like to be an adult. Even though I’ve always been very responsible and independent even when I was a teenager, I never take life too seriously; I have a glass half-full kind of mentality. I know 24 is still young and believe me when I say that I’m in no hurry whatsoever, but I’m just curious to know if there ever comes a time when it just strikes you, when you have an “oh crap, I’m an adult” moment. How did it happen for you? Was there a catalyst? How old were you? Or is it really all about how you look at life and how you feel on the inside, as suggested by Ol’ Blue Eyes in “Young at Heart”?