It turns out that at some point, we are all expected to "grow up" and start acting like adults. Of course there are those who choose not to accept this responsibility and continue to pretend they are still Peter Pans, but in general, it is assumed that once you reach a certain age, you should act accordingly.
Also, I think you start to realize you are making progress in the whole growing up thing when you start to reflect on your past actions as well as perhaps regret a few of them.
In my case, there hasn't been too much of a leap from irresponsible to functioning member of society. Of course I had the weeks when I didn't do homework, slept until 2 in the afternoon, and used an entire package of paper plates and plastic utensils just to avoid the dishes. But I did pay bills and somehow managed to get out of school holding a degree.
The things I reflect upon are mainly my emotions. Perhaps I will reach adulthood when things not going exactly as I plan doesn't make me see red and cause my bottom lip to poke out. It's true, I'm 21 and it still happens. Or maybe it will be when I stop fighting back tears when someone doesn't like my idea.
Pouting seems to be the main issue here.
Also, I think that once I accept that just because someone is at fault for something, it doesn't mean they have to be blamed for it, I will be in a better position to call myself an adult.
Eating habits probably play a factor in the process as well. At some point, the days of surviving on pizza rolls, ramen noodles and skittles must end. Cooking should involve more than just "assembling" and the "pull tab on box" directions should decrease considerably.
I'm not sure when I will completely reach adulthood or finish growing up, but I do know that I've got a long way. I know a few of the personality traits that I could live without though so in the meantime I'll be looking for ways to shed those.