I don't see anything wrong with you being at the high school party. For one you are not that far from being a teenager yourself, besides there are many 21 year olds that date teens.
Age is just a number. I am not saying that a 50 year old should be with a 18 year old, but who am I to judge if a 30 year old dates a 20 year old? There is 12 years difference between me and my wife, and we are very happy. The age difference isn't even noticeible when you see us. We both look like we are around the same age and act like we are too.
Many girls like older guys. I know of quite a few high school girls that only day guys over 20. Just like you don't like high school girls, a lot of them feel the same way about high school boys. It is all about matureity. I read a study once that said women mature fast the then men do, and that a 21 year old woman is on the same maturity level as a 27 year old man. So I guess that would mean that a 21 year old guy is just as mature as a 16 year old girl. In therory that is.
No should you want a 16 year old? No. Now that is sick. But if she is of legal age and she is interesting in you too, there is nothing wrong with it. Like I said age is a number, if you are on the same level in maturity or really like each other, then I see no problem with dating an 18 year old.