I've already written on the surrealness of being called "Dr". Although I felt pretty darn grown up when I was first called Dr. Reaction (and not just in jest by The Boss or Coworker), I've topped that "omigod, I'm an adult" moment.
I'm making my first payment on my college loans this month. Now that is a true "I'm an adult" moment! (a feeling I can enjoy every month until 2016 apparently!)
When was your first real "crap, I'm a grown-up" moment? Defense? First house? First pet?