Well, I think the entire thing is the concept of a mixture of a few things:
First you have the entire appeal of being older, and for a lot of teens, that's a turn on.
Next you have the fact that that's right at the very hormonal 'crush' age.
Third, you have the fact that its forbidden.
And for some people, the entire 'punishment/superior' thing.
But this isn't anything new. from 'Hot for Teacher' to the Athenians this has happened.