Hmm. Good point. But then, part of the reason these relationships work (if they do) is that girls mature faster than boys. That and, I, as a 40 year old woman cannot imagine myself, personally with an 18 year old, no matter how exceptionally mature he might be. But when I imagine Snape with Hermione, I'm responding as my adult self to a guy about my age, as much as imagining myself falling for the same guy when I was 18. Hermione's the stand in for us.

That said, I have seen a Snape/McGonagll fic recently that isn't too awful. I'll have to dig up the name and rec it. It's the same age gap there and he's apparently been attracted to her since his student days.

But I imagine if you really want to read a well-done such fic, you may have to write it yourself.