I've dated someone who was 3 years older, and I've dated someone who was 3 years younger. It doesn't really make a difference as long as you're both happy and legal.
Speaking of legal, I've always wondered: It's illegal if you're 18/over and you go out with or marry or w/e with someone who's younger, right? What if it's, like, just a couple of months difference (just one or two or so)?
Also, I'm 14 so I don't really wanna get into the younger realm because they wouldn't have even started puberty. >_> As for older, I'd go two years up at max. Three is weird because my brother is three years older and then going out with someone his age is o_O
@Revo: I've only had one girlfriend and I haven't gotten any more since then. Unlike you, it's just because of my incompetence that I've failed.