I know what you mean, I'm 22, and I feel pretty much the same way. I also don't really feel an attraction to girls who look like they may be in high school. I just naturally expect them to have interests I think are really stupid or something like that, and it really lowers my interest.
I'm sure I'm being way more judgmental and/or generalizing more than I have a right to. It's not like I'm a genius or have refined tastes or something. I guess it just doesn't take very many years for your interests and life priorities to mature and change drastically. Five or six years doesn't seem like a long time, but the mind of yourself at 21 and yourself at 17 make you pretty much an entirely different person.