I agree with a lot of previous posters that I'll always feel a little young at heart (I mean, I like diapers and watching My Little Pony), though I do feel "older" now. It happened for me after I turned 21. Yes, I was able to order alcohol in restaurants, but that wasn't the reason. It was the following summer when I spent six weeks studying abroad in Ireland, wherein I organized my own personal weekend visiting London. Then, before I turned 22, I got an internship with an Hollywood author and handed so much of his "busy work" like making deliveries, talking to his publishers, and writing my own interview article for his company newsletter.
Afterwards, when I graduated, I realized that the Ireland trip and the internship had given me a confidence to be responsible about my life and my work. I felt like I could take on challenges, which is very comforting in my current postcollege job search.