We Lost One of Our Cats Today...

[Kirby]

My condolences.

[Crystalmaster Mike]

My condolescences.

For about 3 months we (ergo, my mother mostly) took care of a relative's female dog. She (the dog) was 16 years old, older than my brother ánd our previous car. That comes down to about 112 dog-years if I'm right.
So, naturally, she didn't behave like a puppy, only when my mother got back from... where-ever she went the dog couldn't follow her, really. She hang onto her like a toddler.
But she had a heart problem and heaps of fluid in her chest (and in a paw too, eventually), so we had to let her go. One day the vet said she would last another month... the next week, she only got a week or so (less, IIRC). So it may have been for the best.

Anyway, I hope you can enjoy your cats' company for another long while, but keep in mind that nothing lasts forever, except memories.