Why would a man date/sleep with someone 20 years younger than him?

I'm 51 and I can't imagine myself doing it (hypothetically if I weren't married). That's something I never fully understood myself. The best I can come up with is that men aren't much different than women in that they need to still feel like they are attractive and desirable as they age, and what better way than to have a younger person find you attractive enough to sleep with you. That's just a theory of course, but it makes sense. And don't forget...there are women who do it too.