For me, I see it like this

up to 5 years younger = probably a very good and sound idea
up to 10 years younger = might have some differences in our thinking, more than likely would not go younger than this, sex would be great especially for me because she should have plenty of energy or at least I would think so
up to 15 years younger = not likely, it would have to be a really unusual situation like......., and I would probably have to get extra life insurance in case she screws me to death , but more than likely a relationship with someone this young would more than likely not last. She would have to prove me wrong.

Up to 5 years older = not my preference, but I would definitely do with the right person, more than likely would work out without too many age related problems
Up to 10 years older = still a possibility but only if she can handle having sex still, would be nice though because more than likely she would be into conversation which I enjoy anyways. But she needs to "not" look older than her age.
Up to 15 years older = that is probably a one in a thousand shot. Not saying it could not happen but more than likely it would not, there would be one condition, she would not be allowed to put her false teeth in a glass on the nightstand next to us though