I have never had a relationship with someone who was older than me. Most of the time my own age group and up were just unequivocably not interested in me, except for in situations like what Ste11aeres describes-
When I was 26-27, I received a lot of unwanted attentions from guys like that in their fifties or sixties. Since they were total strangers, and didn't know me as a person, it was clear that it wasn't because they were won over by my awesome personality. The thing is, people always thought I was younger than I was, so rather than knowing they were interested in a 27 year old, they probably thought they were interested in a 19 year old.
I have had the exact same thing and it always creeped me out because it was so obvious that it had nothing to do with my personality or who I was as a person at all. However, having said that, all of my relationships have been with men who were at least 5 years younger than me, aside from a (nearly) high school romance with a man who was my age, I was a month older than he was. We lived together for a few years but inevitably failed in part because of his drug use. I also was married once and that man was 24, I was 33. The hilarious thing is that he was the one that was more erm, savvy I suppose you could say, and he was the one that took advantage of me. So I regretted that marriage a lot.
I have had other relationships with younger men but that is exactly the crux of it, it was never because of their age but in spite of it and the bigger the gap, the more I agonized over whether it was appropriate or not, no matter how well we got along (but definitely not in a sense of teenagers, that was never an option). I am 44 now, and I seem to get along better with guys who are at least 5 years younger, but I wouldn't consider being with anyone too young (20 years seems pushing it) as that has implications that I'm not comfortable with. You can be friends without taking it into romantic/sexual waters.
I believe that chronological age is not always indicative of emotional maturity, and I think it is fair to say that I am emotionally asynchronous, intellectually I am mature but emotionally, no, I am behind (although it is difficult to say how much exactly) and that makes sense now that I know I am on the spectrum. With relationships, it becomes unacceptable when the two people involved are not on equal footing. That is why there are ethics involved where a profession or a physican or etc cannot date students or anyone they have authority over, and it's one of the times that I agree with society. But I also don't think it is as easy as gauging just by chronological age.
There are many men that are younger than me (I can think of one of my friends in particular, he is 31) but who have a lot more 'natural authority' or social/emotional savvy. And there are probably men older than me, but I surpass them in that respect. I wouldn't mind dating someone my own age or older at all, but only for the right reasons, which is the only reason to date anyone really. I want healthy relationships above all else. Power imbalances, no bueno.
Like Ste11aeraes I also look a lot younger than I am supposed to, and I kept getting carded until into my late 30's (which is kind of a trip when I think about it
). Babyface McGee or something.