Once you get to about 20/21 I don't think age difference is such a big deal.
Though when you're younger, there seems to be some unspoken rules about girls going out with younger guys in comparison to guys going out with younger girls.
Like; If a 17 year old girl went out with a 15 year old guy it would generally be frowned upon, but if a 17 year old guy went out with a 15 year old girl it wouldn't.
Though saying that one of my friends is 20 and has an 18 year old boyfriend....
But at the end of the day, age is just a number and when two people fall for each other, it never matters. =)