I was not much older than you are when I formulated this theory: 1 in 10 guys is worth dealing with from the time he's about ten, 8 in 10 guys aren't worth dealing with until they're at least 25, and 1 in 10 are never going to be worth dealing with at all. By "worth dealing with", I meant mature. I always enjoyed older people, not just older men, and I married someone 8 years old than I am.
Here is the problem: When you are 20, there is a huge leverage difference between you and a man who's 8 or 10 years older than you are, let alone 20 years older. He just knows more. The problem is, he doesn't know everything, he knows some things which give him an improper advantage because you don't know them yet, and you aren't far enough along to know what he should know or do, but doesn't. If he doesn't know it, it can be worse than if he does. He'll take advantage and not even know you're not as saavy and consenting as you both think. He may mean well, and still do you wrong. When I was not much older than you, I dated a guy who was 12 years older than I was. Not a good idea. I was very mature for 21, but not mature enough for someone going on 36.
In other words, at 20, I wasn't ready for a 40 year old, and most 20 year olds aren't. Oh, have I known some very bright women in their 20s that learned this the hard way! He's got all the cards, and he may mean well, but he can't make it into a real card game. Care to date an associate professor who's "only" 10 or 15 years older, anyone? Can it work? Yes. Can it end in a very very ugly way? Oh, yes. Oh yes, oh yes. Folks, stand back, and don't try this at home! The exceptions practically prove the rule.
I think that even those who are actually ready will have the sense to wait for a bit. You don't know whether you're the exception or the rule, except in retrospect. Not even experienced people do. Guessing wrong about yourself is a very bad ride. Guessing right, and you're still not looking to have someone who can do anything like mutual care when you hit 65.
When you are 30 and a 50 year-old is interested in you, that will be a different story. If you are still so inclined, you can bat anywhere in the big leagues when you're 30. Until then, keep it to within 5-6 years until you're out of college, and no more than 10 years for several years after that.
In the meantime, those guys your age will eventually catch up. Wait at least a few more years before you give up on someone close to your own age. Maybe not many will have caught up with you by then, but you only have to find one. If not, well, you won't have trouble at 25 or 30 finding a 40 year old who's interested.
A particular caution: many a 40 year-old man will want a 20 year old when she's 20, but wouldn't want the same 20 year-old when she's 40. There is a woman in his wake, possibly more than one, who gave this guy the best of her 20s, and deserves much better than to be traded in for you, now that she'll never see 20 again. If he's 40 and a bachelor, don't think you're a match for him until you're quite a bit older. Might you be ready to lock horns with someone who's been around the barn that many times? Maybe. But maybe not. If not, you have a lot to lose, and seeing how that is to be avoided will be something you see in retrospect only. Want to gamble? You only live once? Proceed with caution, and expect you could lose the farm.