To be fair, I'm pretty sure I'm not really an adult, but I think I can pinpoint the time when I was no longer a child. When my father passed away, my mother and I were visiting her family in another state. My older brother (who had come home to find my father dead of a heart attack) had called my aunt's house to tell us what had happened, and my mother sort of broke. The part I distinctly remember was sitting downstairs with her, while her sisters and nieces and nephews milled about upstairs, and having to convince her that it was true - she's always been a very practical person, but there was something about this news that she just couldn't handle, and she kept insisting that it wasn't true, that "God wouldn't do that to us." I had to convince her that no, Rich wouldn't have called and told us Dad was gone unless he was - Rich was already an EMT at that point, and the fire department had already come and confirmed what he'd found. The only thing I wanted to do was cry until I couldn't cry anymore and have Mom hold me and tell me everything was going to be all right, but instead I was holding her and telling her we were going to be all right, even though I didn't believe it.
On the more frivolous side, the first time I actually felt like a grown-up was when I was able to make Mom's flour tortillas myself, and have them turn out edible. The fact that I could do something I'd watched my mother do so often, and not screw it up? Hell, that wasn't just being a grown-up, that was being AWESOME.
So, what about you guys? When did you feel like you'd become an adult/stopped being a kid?