In Bossy Pants, Tina Fey explains that most women, when asked to identify the moment, usually realize they have "become a woman" in some relation to men (either they were hit on, or kissed, etc.). I don't know exactly when I "became a woman." I appreciate the recent movement for ladies above 18 to be called "women" and not "girls," but I would (myself) rather be called a girl? I guess in the comedy community I don't know, like, ANY people I would refer to as full adults. I call "men" guys and "women" ladies or gals.
I do, however, know that when I was in high school and SNL did a cold open that mapped the FRIENDS finale over George Bush and Dick Cheney in the oval office, I had to explain what all the jokes meant to my mom. I think that was a decent mile-marker entry into adulthood.