i think it really can depend on your culture. in some places in the world, it really doesn't matter what age you are as long as you're in the right social rank or whatever.
i have friends who are married and twenty years apart. i think they have a great marriage too. :)
they weren't going to get married though if they didn't have the woman's parents' permission because to them, the age difference was a problem.

personally for me, i don't want to go out with anyone younger than me or too much older than me. even though i'm 21, i still feel like there are lots of ways i need to mature and i wouldn't want to feel like i was being looked down upon in a relationship because i was the younger one. at the same time, i wouldn't want to go out with a younger guy (like, anyone two years younger) because i wouldn't like to feel like i'm dating a guy who could be one of my little brother's friends.
i also want to add that my parents are only 2 months apart.

overall, i think it's one personal choice whether they decided to date anyone older or younger than them.
to some, it's only a number. to others, its a deal breaker.