I don't have "rules" per se, but being only 24, there's only so low I can go.
I'm still young, but I'm old enough to be at least somewhat of an adult. I definitely wasn't at 18, 19, 20, or even 21, by my own admission. The top of that range is kind of my limit -- and it's going to depend on the guy.
My man is actually 2 years younger than me. We started going out when I was 23, and he was 21. And that is absolutely as low as I'd go, because at this age, a couple years makes such a huge difference.
He is uniquely mature; in certain ways, possibly more than I am. I don't notice that he's younger much, when I'm just with him.
But his friends? That's a mixed bag. They're all nice people, but there's some of them that I feel aren't quite in the place in their life I like in the company I keep. I don't make a fuss about that or anything. They're his friends, and they're all decent enough people, and I don't mind hanging out with them now and again. But the age gap shows when it comes to hanging out with each other's friends, even though it's 2 years difference.