How big does the age difference have to be before it's creepy? The formula I've heard is half your age plus 7 years. If you date someone younger than that, you are officially a cradle robber. Seems reasonable to me.
Is a 40-something dating a 20-something creepy? Not as long as they share the same interests, desires, what-have-you, but I can't imagine there's going to be a huge overlap there.
I actually think this comes down more to personal preference. At 36, that formula outputs 25... now, I know a number of 25-year-olds. Nice enough people, most of 'em, but at my age I sure as hell don't want to deal with their issues. I certainly wouldn't want to deal with myself at that age, knowing what I know, except maybe to smack some sense into him. And I suspect Kuildeous won't likely want to deal with the issues of someone twenty years younger, except as a fling, and flings can be dangerous things indeed.
Seriously though. The story of how I met my current wife and why my first marriage lasted effectively a year is this gigantic red flag saying "Do not take dating advice from this guy."
What are the stories?
If I have time some evening I'll tell you the short version. The ultra-short version is that my (current) wife and I, both very intelligent people, made some impressively, stupendously, un-fucking-believably dumb decisions regarding the expression of feelings, back in... 2001 IIRC, which, hey look, it's 23-year-old me! Seriously, I could smack that idiot. Actually, a big hug and a stern talking-to, of the sort I give to my five-year-old, would likely be a better idea.
Here, have some sappy but relevant Indigo Girls harmonies: