It would depend on who it was. I know a couple of absolute darlings in their mid-20s and mid-30s. The age gap doesn't feel inappropriate as such, but I tend to think ahead and wonder what would happen when they were at their sexual peak and I was withering away. And I wouldn't want to die on anybody - the relationships I've had have generally been quite intense and closed, so there's usually been a tendency to ignore the rest of the human race (why bother socialising when you're not lonely?), so losing each other would be a huge readjustment. I'd have to talk it all out with the lady in question, and see if she had any solutions to those problems, because I don't. So, as a rule of thumb, I just assume they'd see such an age gap as absurd, as long as nothing happens to make me quetsion that.
I once had a relationship with an older woman (about 20 years older), and I never felt very strongly bonded to her.....I still don't know if it was simply her age or the fact that we always knew we'd never have a fully-fledged relationship - she was married to a promiscuous man and didn't want to leave him. It was great not feeling vulnerable to her, but I guess I'd feel threatened if I had a partner who didn't feel attached like that. On the other hand, I've heard tales of older men being total bastards to young partners, and getting away with it, which kind of suggests they can get quite strongly attached to these old farts, otherwise why not just leave?