Ugh, this topic is quite relevant for me now!! I have always dated older men, although the age range has usually been more like 6-8 yrs older, not 20... 2 years ago I got seriously involved with someone who was 19 yrs older than me. It didn't really work out, but we are still friends. I have gone out on a few dates in the past few weeks with a new guy, who is about 15 years older than me. So far, it seems to be going really well. I'm in my mid 20s.
Like some posters have said, I am attracted to the stability of an older man. I'm sure I'd be attracted to a younger man with this same quality, too, but it's rare at my age. I don't need a provider (I will graduate next year with a professional graduate degree and will make good money), but I'm incredibly attracted to someone who seems "settled" and to "have it together." Also, like another poster wrote, I do have something of an old soul - even as a child, I preferred to hang out with older kids and adults. That's not to say that I don't love hanging out with friends my own age, and I do, but... I am just not as attracted to men at that age. With the older men I've dated, I've not felt like a trophy - we legitimately get along better (maturity-wise). Maybe it helps that I'm not so much into pop culture or things like that.
I fear, though, that in 10 years or so, a man that much older than me wouldn't be able to keep up with me, and that concerns me. I want to have children eventually, and I worry about doing that with someone who will be in their 60's when that child graduates from high school. I worry about being widowed young, or having to nurse an elderly husband, while other people my own age are out "doing life" with their same-aged spouses. These fears tell me that I am definitely not ready to marry any of the older men I have dated, but it's a bit of a struggle for me - do I refuse to date older men, for fear of getting involved, and then married, and then facing these problems, or do I try to force myself to meet men my own age that I'm not really very attracted to? Also, I worry about societal judgment and judgment from my family and friends - this would not stop me from being with someone I loved, but I think it would be very wearying to constantly have to feel like you had to explain yourself, or to feel that strangers were judging you.
I know very well from personal experience that life is short and precious, and can be stolen away at any moment, so maybe it's better to overlook the concerns listed above.
I do wonder what the correlation is between women who date older men and women who didn't have a father figure growing up - I didn't, and I wonder if it plays into who I'm attracted to. I think it must in some way...