Does being a cougar count? Okay, in seriousness... I am a woman in my early 40s. I like to think I look pretty darn good though, and people tell me so. I don't specifically go after younger people, but both my boyfriend and girlfriend (yes, they know about each other and we're all okay with it) are in their mid-20s.
In my case it's just.. I don't have much in common with people my age. I'm a gamer, an anime fan, never been married or had children. I simply have much more in common with people in the 20-something age range than I do with people my own age, so those are the people I spend more time hanging out with.
It's not like I'm trolling bars looking for someone 'young and fresh'. Or that I necessarily prefer someone that age. So long as they're legal and not about to drop dead from old age, it doesn't really matter to me. It's just that, because of my hobbies and interests, most of the people I wind up hanging out with are a good 15-20 years younger than I am.