It really depends on her maturity. However, it is likely that she isn't mature enough for you. She is probably living the life of a early-twenty-something, which is probably different from how you are living your life. It all depends though... I've seen 20 year olds who settle down like old women, and I've seen 30 year olds party like they were 21.
However, on average, I'd be careful before I started having too many feelings for this girl. Have a great time, and if it evolves into something more, then great! Try to keep up with her by going out with her on the weekends (bars/clubs, at least one day on the weekends), and don't talk too much about marriage or kids (if that is where you are looking right now). In fact, if the relationship works out, let HER be the one to bring up those subjects.
I'm 26, and I'm dating a 23 year old girl who just graduated college last May. I have a hard time keeping up with her. Is your girl in college? I started dating my girlfriend in Febuary of her senior year, and the couple of months we were together while she was still in school presented some unique challenges. Fortunately, I am a grad student, so I wasn't "that creepy old guy" that came around. I was able to blend in. However, for you I would imagine that it would be more difficult (no offense).
The age difference will become a lot less significant as you get older. However, over the next 5 years or so, things could be rocky (assuming you are looking at this long-term). My best advice is to be flexible and be willing to go out and have a good time, rather than stay in or go to the movies. Remember what it was like to be young, and be ready for some wild nights.
Good luck, and have fun!!!
R