Cats get old so you need to take extra care of them through regular checkups by a vet. A typical checkup for a geriatric pet will likely include a complete physical examination, a blood test to check white blood cell counts and thyroid activity (among other things), and a urinalysis.
The goal is to verify a cat’s health and hopefully detect any potential problems early. Just like people, the older a cat gets, the less its’ immune system is able to fight off illnesses.
Consider getting a heated bed, a litter box with lower walls, and special cat food to provide the nutrition your elderly cat needs. With proper care, you should have your cat around for you to enjoy its presence a little longer.