Here's a non-exhaustive list of times the thought, "So this is adulthood," struck me pretty strong. Without further adieu, in no particular order:

Scheduling my own doctor's appointment and having to fill out all that obnoxious paperwork.
Going pants shopping and purchasing them from the women's section instead of the junior's section.
Buying the women's jeans and being excited about how well they fit and how much coverage they provide.
Wanting sneakers at Firefly, despite them not going with my outfit. However, I still packed like a kid and went with the cute shoes instead.
Having people trust me enough to work from home.
Dealing with the bullshit that is car trouble. Times three.
Having to request time off to go home (well, now my parent's house I guess) and not just being assigned a time to leave (like college breaks).
Needing to recruit a chaperone when I needed my wisdom teeth out, rather than just having my mom around for it.
Giving a presentation about my job and being thought of as the "professional opinion." Then doing a good enough job to be asked to do it several times more.
Handing out business cards.
Being included on a recipe email chain, and actually having something of merit to provide.
Taking over the title on my car AND paying my own insurance for it, which is also in my name.
Going to my mailbox to receive nothing but bank statements and bills, rather than care packages and holiday themed cards.
Laying in bed all night stressing about work.
Laying in bed all night and not being able to whine to my mother about it.
Things breaking around the house and not being able to just leave it for my dad to fix.
Missing holidays, birthdays and other social outings because of work.
Spending money on things freely and not feeling terribly ridden with guilt for spending my parent's money frivolously.
Filling out my own taxes (albeit with a lot of help from my dad).
Having to pack my own lunch everyday.

Were there any times that you were ever overwhelmed by feelings of adulthood? Leave 'em in the comments! :)