Hi, L. --
I am such a fan of old cats. My gal Kalinka will be 17 this year, and she's still kicking butt and taking names. My husband's cat from childhood died at almost 24 years old! So, while 18 is definitely "old," it's not unheard of for cats to live well beyond this age.
I am a volunteer at a no-kill animal shelter where we have over 600 cats. Now, I say "no kill," but we are humane and will put an animal to sleep if we believe it is suffering. That said, we do everything we possibly can to support our animals until they tell us it is their time to go. We know them so well, it is not difficult to see when they have decided they would like to pass on -- generally demonstrated through lethargy, refusal to eat or lack of desire to eat or interact.
We also have an entire collection of cats who are blind, deaf, incontinent, missing limbs, have neurological issues that make them "wobbly bobbly" or who have been abused in various ways. We have cats in renal failure, cats with kidney disease, cats with Feline Leukemia and FIV. And the same rules of life apply to them... if they are not demonstrating that they feel too miserable to continue on in life, we honor them by helping them manage their medical conditions or other needs.
So, I wanted to mention this whole range of things because often people feel that animals probably want to cross over if they are not enjoying the same kind of life that we as humans would want. But, I think it's important to remember that animals are animals - if they are loved and supported, cared for, comforted, fed and kept in a clean environment, they don't worry. They don't care about tomorrow. And if they are not in pain, they're generally going to want to stick around to be with you.
So, with your old gal, maybe an approach you could take would be to restrict her to a smaller area than her current range - somewhere that she can feel comfortable and safe as her vision and hearing decline. She's got to feel tremendously exposed, and that would certainly make her nervous. For our shelter's blind cats, we try to keep everything as consistent as we can for them so that they can orient themselves and feel confident. With our deaf cats, we try to not startle them and offer something of ours that they can smell if they're sleeping so that they know we're going to touch them. Things like that make the cats much more relaxed. Nobody likes to be startled or lost!
I have seen remarkable resilience in cats and recovery from some of the most unimaginable abuse and misfortune, disease, and other conditions. So I have huge faith that, with love and some "putting yourself in her shoes," you can help your Tinker be more comfortable. I would not put her to sleep unless she's not eating and demonstrating that she's had enough of life.
By the way, my girl Kalinka cries, too. And I can see by the graying of her eyes that her vision is probably starting to wane a little. If it were to get worse, I would do just what I've suggested to you -- make her space smaller and "safer" feeling so that she would get comfortable with being able to navigate freely.
I wish you the best. I know you're leading with your heart. Let me know if you want any other suggestions or support. I am a HUGE fan of the felines and the canines!
I thought of this after my initial post:
One of our shelter's blind/deaf cats, named Terry, also cries a lot. But his weight has been constant and he's been healthy for years. We believe he's just wanting to make sure he gets the attention he craves so much and doesn't want to be overlooked.
Weightloss is one primary indicator that a cat is having health problems. We tend to use 1 lb of weightloss in a month as an indicator, though we track our shelter cats' weight and monitor any continuing downward trends. The smaller the cat, of course, the more we look at the ounces lost, not just the 1 lb/month rule.
Cats in pain will often hiss at you, or you will notice mood and behavior changes. You may also be able to determine that an animal is sensitive in some area, indicating some pain there.
I hope this helps!