I remember Katherine learning that she was ageing faster because of everything starting to catch up to her, but I always got the impression that she would have continued being physically healthy if Silas hadn't of drained her of nearly all of her blood and passed the cure into himself. That would fit with the cure only having enough for one, and two people not being able to both take the cure and survive, otherwise Elena could take the cure and let Damon drink some of it from her

I would have expected the characters to discuss the cure in terms of it having a catch and it was a good thing that none of them took it, something like that, if the cure was supposed to be what was at fault for Katherine's ageing, instead it is still being seen as a viable option for them, so I would imagine anyone taking the cure would be fine just as long as it remains in their system