If it's okay for boys to date people younger than them, why can't any other gender do that? It's okay, as long as you and this person are mature enough, that's fine. (And, as you get older, asking as you're not an adult with a kid, because that might not be good.)
I've never asked someone out. I've liked the same girl for over a year, though. My best friend, which makes it even harder if she says no (when I'm old enough, because I don't feel old enough or mature enough).