1. Feed your cat a high protein/high moisture diet. Yes, some veterinarians still subscribe to the lower protein prescription diet they can sell you. They might truly believe this is an ideal way to ease the burden on senior kidneys (and they also make money selling you this food). I think that belief is fortunately not as popular now, since most cats I met who had to eat that food really hated it. I advocate a raw food diet for cats who enjoy it and can digest it. If your cat won't touch it or vomits it up, stick to canned food and supplement with digestive enzymes and probiotics. Canned, grain free food is always preferable over dry food since your kitty needs the moisture to keep this kidneys hydrated. Adding extra water to make the food as soupy as they will tolerate is helpful too.
2. Build your kitty a catio! My darling orange tabby Roy was the last free-roaming cat I will ever have. He joined our household when he was about 9 months old and even though he was neutered, he acted like a Tom cat. He was very unhappy staying inside and I didn't have a catio at the time. When I later had a catio, other cats he disliked occupied it most of the time. My point is that I worried about Roy every single day he went outside. He always came in at dusk, except the first warm night of spring every year. When he was diagnosed with diabetes during the last year of his life, he would run inside at "shot time" because he knew I was taking care of him. Roy passed away at 17 1/2 YO from the pancreatitis which caused his diabetes. We were very lucky he was never hit by a car or attacked by a dog or shot at, or any of the horrible things that can happen to a cat alone outside. Now we have several catios that are basically bear/eagle/coyote/human proof. I don't have to worry about the kitties getting the sunshine and fresh air they need.
3. Rethink annual vaccinations. There is a ton of information on the Internet about the dangers of vaccinating animals every year. I personally know cats who got cancer and had to have amputations, or suffered twitching/tumors forever after, and most commonly, are sick for days afterwards when they ere perfectly healthy before the shots. Obviously if you have a kitty who is outside a lot, such as a barn cat, you would want an initial rabies vaccination. But for a cat who lives a strictly indoor life, consider the risks of vaccinating once they are grown. And never vaccinate a cat who is already sick (yes, I once a vet try to do this on a kitty with a high fever).
4. Choose a "Muttigree." Purebred cats have more health issues, from my experience.
5. Sadly, cancer is a common killer in companion animals and we don't really know the role chemicals have in contributing to different cancers. Avoid as many chemicals in your home as possible, such as scented litter, fabric softeners and air fresheners. Your kitty is likely in your home 24/7 and breathing those chemicals into his tiny little body at a much higher concentration than the average human. If your cat's litter box stinks, chances are it needs to be cleaned! There are many good natural and clay-free litters available now. Buy a scoop for each box, keep a little covered waste can, whatever you need to make it EASY to clean a few times a day. Also, open the windows to ventilate as often as possible and keep healthy houseplants that filter the air.
6. Help your kitty with extra steps/ramps and rubber backed throw rugs if he has trouble getting up on furniture or slips on the floor. Keep heated beds in multiple locations to soothe his aching joints (kidney kitties also seek heat to help themselves feel better). Don't let your cat become obese in middle age, which may later translate into joint problems. Keep your senior kitty's nails trimmed so the nails don't grow into the pads or cause his gait to be off. Help him stay groomed with brushing or combing more frequently, since older cats often don't groom themselves as frequently.
Sure, young kitties are cute but I enjoy meeting senior kitties, who often have unique preferences. If we are lucky, we all get to grow old.