I think part of the appeal is seeing a woman in that situation who is the more "in charge" of the pair. Not, y'know, domineering or "the one who wears the pants, nudge nudge." Just that she's the one coming to the relationship with a little more life experience than he has, but also that she isn't seen as some kind of cougar.

I honestly can't think of another relationship I've seen like theirs.

That said, I'm ambivalent on the romance, but I love the relationship.