I think the older you get, the less important an age gap becomes. It also depends on the people - some people are mature for their age, and some people are always young at heart. Also, people go through different stages of their lives at different points, depending on whether you spend a lot of time at college, or whether you have a career, or whether you just want to start a family straight out of school.
Social convention also makes a difference. Age gaps seem to be a bigger deal now than they used to be. I mean, because of their generation, all of my grandparents married exactly ten years apart, with the man ten years older than the woman, but, for my parents, who met at university in the 70s, where they were both in the same career (medicine), the age gap is only a month. All those marriages have been strong since the day they said, "I do," so, of all the factors that affect a relationship, I don't think age is one.
(Personally, I go after people waaaaaay older than me, so I may be biased on this issue. =3)