Oldest I've dated... 26, when I was 19? Youngest, a boy who was a year younger than me. I don't think I'd have trouble dating guys 1 or 2 years younger, though at this age you can really tell how most guys are really immature in comparison to girls my age, which is why I'd say the ideal age for someone who's 20 like me would be a guy who's 25-ish (though it always depends on the guy)
Nowadays I see friends of mine dating guys who are 29 even 30, and quite honestly, they don't seem that old. Personally, I'd still be intimidated by the fact that I'm with someone who's nearly 10 years older than me and supposedly knows a lot more about life than me, but then again, it depends on the situation...