Mark, I don't believe there are any written rules about it. It really is about being attracted to one another, at any age. Nowadays it doesn't seem to matter all tha much. Years ago, older women would not think about dating much younger men, today it happens a lot and I think people are attracted to each other for many and different reasons. Years ago, also, it was thought that if a very young woman married an older man, she was likely to be after his money and/or position.
So, what about your age? It just depends on what you're looking for in an ideal mate. You can certainly date women from say, 19-20 years of age and can date women who might be a little, or a lot older than you, but it depends on how comfortable you both feel about it. You've got to decide what your comfort level is with someone that might be 35, 40, or even 50, but the older you go up the scale, the less likely I think that women will want to date you. I don't know. Let others chime in. There was another thread going on around here in the last few days that discussed this as well, and for what I saw, most women are most comfortable dating men 5 or 6 years either way, some even changing their profiles to reflect this, as opposed to their previous choice which was about 9-10 years either way. Younger women were complaining a little about the level of maturity of younger guys, and certainly, it sounded as though that the women who are in their early 20's don't want to date men younger than them, but also don't want to date men which are much older. So, without understanding your motive for this question, really can't say.
Good luck in your search.
At my age I'm really not interested in dating guys younger then me because guys that are younger then me normally have much different interests then I do. As I get older I find I have more in common with men that are older then me which is what matters in my mind. To be totally honest I don't think I'd date someone more then 10 years older, I feel that someone older would have so much more experience then I that wouldn't feel equal.
Honestly I think that it's more about what you have in common, not how much older/younger you are.