Cats can live well into their 20's in some cases.

You didn't describe anything that sounds like the old man has a shitty quality of life, so ... not sure why you're thinking this way?

and doesn't seem to be in pain, he's probably still enjoying life. Check with the vet.

And as long as he seems comfortable. Has he been checked for hypothyroidism? That could explain
his thinness, especially since he is eating well.

He sounds content in his old age. I had a cat live to 22.

But that was my thought too, since his appetite is healthy. Easily treatable. They give you a prescription cream that you rub in their ear.

I had it backwards. Yes, the cream works well.

if there is arthritis, go for the smallest possible dose. Tooz. much is hard on kidney or liver, especially at that age

That's the question I always ask. Or as my daughter put it about our Best Cat Ever when she suggested it was his time, "He's just not having fun any more." I'd take him to the vet for some blood work and a general checkup before I made a final decision. He may still have a couple of good years left.

about putting her asleep, but the next day she acts and looks better. This summEr she was low on vitamin E and was pulling her hair out. Vet told me to give her vitamin E everyday and she started to grow her hair back. She looked like a poodle. I know her life is almost over, but as long as she is not in pain I will wait to make that horrible decision.

So long as appetite is good!

he has 3 litter boxes close by, where, thank goodness, he pees and at times does poop there. Cat urine is so much harder to clean!
I complain to him everyday but he could care less. So, at least I get surprised when, at times, he doesn't poop outside the box.
He is also so mean he can't be with my other cats and the one kitty who he kind of gets along with, he will at times be really vicious with her but will eventually back down from shredding her. He hates the other cats. He loves dogs.
Anyway, in your situation, imo, I would just let him live it out until obviously uncomfortable, in great pain, and/or unresponsive to outside interests. Just my personal road I would take with an old kitty friend.

About a month ago I had to bring my kitty to be euthanized. It broke my heart. I still feel her presence. She was of an indeterminate age, a shelter kitty that I adopted in 2010. I had to bring her to the Vet with far more serious symptoms than those posted. I miss her but would never think of euthanasia with the symptoms described.
I miss my kitty but am too old to adopt another. I wish I still had my furry friend.
I'm sure the poster has good intentions but with the listed symptoms, it seems too soon.

leave him be. He will let you know when he is ready to just be done. You are doing a great job for him. My thoughts are with you.

paying close attention, which I know that you are, your kit will tell you when he is ready. My litle rescue would spend all day sleeping on my lap if he could, and he is only ten, a real lap cat.

I am so glad to know that you are thinking of his well-being.

to 20 and beyond. It sounds as though your cat is in pretty good shape for his age. Your description sounds very common for an elder cat (or elder person for that matter) who is otherwise fairly healthy. Has your vet suggested euthanasia? Why are you considering it?

If he's eating and doesn't seem to be in pain.

Had a extraordinary kitty who made it to 21. She loved everyone, including friendly dogs.

My wife and I had cat named Sassy that passed back in 2018. She was a month shy of her 23rd birthday when she crossed over. If your cat likes to eat and purrs when you pet him, you're doing well. It's not time yet. When Sassy passed, she curled up into a ball and went to sleep first. We knew something was up because laid down on the floor in the middle of the kitchen. This was something that she never did in all her years. She was breathing but not responding, so we took her in and helped her over at that point. The last cat I had to put down had a stroke. That was blatantly obvious and I rushed him to a vet. He was suffering and the prognosis from the vet was really bad. We said our good byes and saw him over. You're doing the right thing by keeping an eye out for any signs. Your cat will let you know.

"Don't cry because it's over. Smile because it happened." - Dr Seuss

and not in pain, then keep on trucking.