Is there an advantage to the traditional older male/younger female age difference? Fieder and Huber (2007) suggest that there is. In their survey of 10,000 Swedish parents, they found that men maximized their number of offspring when they chose a mate 6 years younger. Women had the most children when their mate was 4 years older. Both men and women chose a mate younger than their first mate after a separation, although women still preferred a mate older than themselves.
Beck and Powell (2000) used evolutionary modeling to determine whether a preference for older males was likely to result; they found that this was only likely when older males provided "more than just sperm" -- when the older males were proven providers.
I've dated all over the (legal) range, myself. My current partner is 13 months younger than I am; I enjoy giving him crap about being "a kid." However, being a transsexual man, I tend to be taken for 5-10 years younger than my actual age. I'm often "carded" for age when my partner is not. FTMs often look much younger than they are, because of the delayed onset of secondary sexual characteristics. Since (like everyone else) FTMs are mostly heterosexual, this means that they often look younger than their female partners. I've heard many partners of FTMs complain that they get weary of the implication of pedophilia that gets levelled at them when they go out with their partner. I've heard more than one case of a date being mistaken for a mother-son dinner in couples like this, even when the FTM partner was older than the female date. While this isn't the only complaint I've heard from partners of FTMs, it's certainly one of the more common themes. Then again, I've also heard FTMs complain about the flirtacious behavior of teenage girls, who more often than not, mistake us for their own age bracket.