my girlfriend is a bit over 6 years older than me, and we're very happy. there was some worry early on, and there's some natural differences between people who are going to be at slightly different points in their lives, but i agree with whoever said that the women my age are generally not as mature and that gets old.
of course, the men my age are not generally as mature, so it works both ways.
my previous girlfriend was about a year older than me, but that's not really enough to create much difference.