I find myself in this for a few minutes, but then I kind of gloss over it.
There is this librarian who is 11 years old than me (I'm 22) who has a five year old little boy (He's quite spritely, to say the least), so all the signs point to "Too young to date", and she herself thought that dating someone younger constituted being a "Cougar".
Cougar rules dictate that you have to be old enough to be my mom in order to be a cougar (so if she were say, seventeen or 18, maybe, there would be an issue), so there are no problems there.
However if you go by X Divided by 2 Plus Seven (For Reference [xkcd.com/314]), I just measure a year and a half under what she can date without being creepy.
But if there is something there, Why should it bother other people? If two people are happy, don't make a big fuss out of it. If it bothers you, you'd have to be pretty insecure, it's the only thing that makes sense.