Your cat really isn't that old. While an older cat will gradually experience some kidney function loss (mine is almost 17 and doing very well) you are obviously having some troubling health issues with your pets.

It could be the just bad luck coincidence with your kitties, but it could be environmental. When I was a child, we lost both our cats to a poison that our neighbour was using in his garden to keep away bugs. So it could be anything in an environment. I urge you to see a compassion vet who likes kitties asap. Good luck!