In any case, regardless of the type of relationship, the golden rule is the campsite rule, suitably generalized to all relationships: strive to leave your partner(s) in a better shape than you found them.

ie. You should dump them as quickly as possible - while there is still a chance that the amazing sex offset the deterioration from aging. (You can stay with them a bit longer if you encourage them to exercize more, eat better and you give them a supply of tretinoin.)