I realized I was an adult when I fell asleep one night and I was 5'2", and the next day I was 7'10". Oh wait, that didn't happen? Hm--no, guess not. I'm still 5'2". Crap.

The next best answer would be: I realized I was an adult when my laundry stopped magically cleaning itself and folding itself nicely in a neat pile in front of my room. Man, I hate laundry day. Never get old if you can help it! :-/