What exactly are you looking for?
If a chick is simply a fvck then anything legal is acceptable... whether you would want to do someone who is just legal is on you as far as whether it's attractive to you...
If you want to make this chick a GF then unless your development has been severely stunted then huge differences will exist between you dating a chick who is high school... Univ changes folks a lot, and honestly if you hook up with a 17 - 18 chick, chances are that by the end of college you will no longer be together because she's still got a lot of growing and experimenting to do...
I'm 23 and I find girls 20 and up to be good prospects... I've met a few 18 yr olds who've led very different lives due to their vocation, usually they're models, and as a result they communicate on the level of a chick who is a few years older... what age girls do I usually end up with? Chicks who are a little older than me...
There really isn't an answer to your question... every chick is different and it depends on so many variables... there's a 19 year old chick that I would date and have as a GF, but there's also 22 year old chicks that I would never, lol.. I dated a chick who was 26 recently, things didn't work out at an early stage but lets say things had gotten to be a lot more, then you may find that she wanted to do the serious thing much sooner because she's getting older, but some other 26 yr old chicks don't feel they need to rush into something like that...