First girlfriend was fifteen when we started going out/sixteen when we lost our virginities. Dated a girl two years younger than I. In terms of gap, I've had sex with people five years older/younger either side (recently--the younger was 18, the older was 28).
I've never been with anyone with a big age gap; my ex was a year and a half (one academic year) older than me and my current boyfriend is about 7 months older than me (I'm one of the younger people in my year).
I have no problem with big age gaps in relationships but I'm yet to meet anyone significantly older or younger who I have a lot in common with or am attracted to.