I'm 22, female, and I don't think I'd date someone younger than me because I probably wouldn't have much in common with them, and it might feel a bit uncomfortable being the "older woman" around friends etc. However, I wouldn't rule someone out just because they were younger than me.
I don't think there's anything wrong with an age gap, just as long as both partners are adults and are at the same point in their lives (especially where marriage and babies are concerned). Other people can't resist sticking their nose in though. My boyfriend is 3 1/2 years older than me but I only look about 16, and we get loads of sideways glances (mostly from women my age!) because the age gap looks a lot more than it actually is!