I’m posting about this for a couple of reasons. One is just to share the experience and the other is to hopefully hear about others in similar situations.
Zephyr turned 21 years old this spring. She’s completely blind, mostly deaf, and being medicated for hyper-T and high blood pressure. Her adult fighting weight was a chubby 11.5 pounds and now she’s a lean, mean 6 and change.
She is very active in spite of the sensory issues. Because she’s blind, the majority of her walking in in circles, but she still knocks out probably an hour a day of brisk walking.
About a year ago she started crashing. Got down to 5 pounds and was very weak. Multiple vets said that even if we could stabilize her, she wouldn’t gain back the weight. We changed her thyroid med to a transdermal compound because pills and liquid were hit and miss. That plus a couple weeks of subcutaneous fluids and a few months of very frequent feedings got her back over 6 pounds and healthy again.
Her quality of life is pretty good. She sleeps well and “chases rabbits” pretty often. She still grooms thoroughly. She can manage wandering the house including a full flight of stairs every now and then.
I guess part of the reason I’m posting this is my roommate’s cat passed a week ago. He was about 16 and had been failing the last few months, but crashed suddenly the day before. I took him in to the vet hoping a steroid shot and antibiotics might bring him around, but it was clear from the examination that it was time.
Someone commented in another thread about waiting until the wrong time to deal with a declining cat and I guess my thought here is that you make that judgement on the basis of what you see. Cats can have acute turns for the worse health-wise that don’t always dovetail with the convenient time of day or day of the week.
Sorry for the ramble. The recent loss reminds me that old scars open surprisingly easily.
Cats are such hedonists. I figure, as long as a cat is eating, they’re capable of enjoying scritches behind the ear, long slow strokes down their back, and a nice rub on the rump (“elevator butt”) as much as any other cat. Your cat can smell you (and thus, sense your presence easily) even without sight or hearing, and still enjoy your time together.
This part:
“…but she still knocks out probably an hour a day of brisk walking…”
cracked me up, though. I pictured the kitty equivalent of seniors doing their morning mall-walking rounds. 
My Himalayan lived to about 17. He began to decline a year earlier, losing his senses, sight and hearing. He practically lived on my lap for months (and he was always a standoffish cat). Finally he went into a real decline, couldn’t do the stairs, stopped eating, stopped grooming. After a week with no change, we made the decision to have him put down. It was a kindness, really, and a very hard thing to do. As long as your kitty is still somewhat enjoying life and not suffering, you all are lucky. But when the time comes, don’t let him linger. There are millions of other cats who need homes, I hope someday you’ll get another.
We actually have some good news for our 16 year old cat with arthritis - we’ve been treating her with glucosamine supplements for a couple of years on the vet’s advice, and her arthritis has actually gotten much better - she’s still not jumping up, but she’s jumping down from things, scampering a little, and her limp has really been reduced (it’s hardly noticeable now). We started treating our younger cat with the glucosamine at the same time too, so hopefully she won’t get arthritis.
How can you tell that she is blind and deaf? Does she ignore your calling her name and hand gestures offering petting?
I know she’s blind because in the brightest daylight her pupils stay dilated. I know she’s deaf because while she still responds to very loud sounds, she can’t localize them.
And after 21 years of having her around, I have a pretty good idea of how she responds.
She does get along surprisingly well with smell and touch. She’s a lot more vocal now and when she’s on my computer desk and knows I’m there, she’ll hook her paw behind my arm and tug several times to let me know I’m supposed to do something. 
I bet she plays a mean pinball though.
I work in veterinary medicine. Occasionally there are clients who bring their 4 lb. elderly, failing cats in for an exam, hoping something…anything…will cure them. I have a huge soft spot for old kitties and I would get so angry at the owners sometimes, wondering why they couldn’t see what was right in front of them.
Then my own 16 year old cat was failing. He lost weight. He got weaker and was vomiting and not able to keep meds down. He would stumble and fall when getting off the bed. One day I realized that *I * was that owner who couldn’t see what was right in front of me. I euthanized him a few days later.