Two animal topics at once? This is getting out of hand.
Cats are a nice addition to a house provided you get the right cat. Most of my cats get to around the twenty year old range before getting put down due to seizures, emaciation, tumors, organ failure, age, or when it hurts when they move. I swear that once a cat hits the senile age their weight plummets to skeletal and they walk around with an air of death over their head for YEARS before they actually kick the can.
The only cats I seem to have a problem with are the fluffy ones. One when I was young was an asshole and clawed everything, another died of a tumor in the intestine at the young age of thirteen, a third died at age five from bone marrow cancer, and the current fluffy cat I have pisses all over furniture and has to live in my basement to prevent that. Meanwhile all of my tabbies live longer than a kid through college and are great to have around. One day I'll learn my lesson.
As for the old cat the thread maker has, simply keep the dog away from it. When they hit the geriatric age, they wont take shit from no one; they also cannot run away if that shit chooses to bite back. Dogs have a breaking point and some, if scratched, will defend themselves with teeth even if the cat is harmless.