Depends on the relationship. If you go for a marriage then the ideal should be around the same age or a few years difference. There should be an exception in case you fell in love with someone significantly older/younger. If I just date someone for a short term relationship or just because I like her but I got no feelings, I wouldn't mind if she were 5-10 years ( not for now I'm 22 :P ) younger or 10-15-20 years older.
I would prefer a slightly older woman but all I attract is younger girls ranging from 25 to 19-18. Why? I find older women are more mature, they have their life sorted out (generally), they are independent, the only downside is they probably want children and eventually marriage as their biological urges kick into overdrive.
I'm impartial really. I find older guys really attractive as they seem more mature and have life on track (some of the time) and are sometimes more ambitious and generally more sincere about things. But I gave liked younger guys too.. Age doesn't particularly bother me, I could happily date up to 8 years above or probably 3 years below my own age..