When has your pet had enough?

Monty is 18 now. Not bad for a feisty little Maltese-terrier cross. More terrier than Maltese. We gave him a new lease on life when we spent a fortune fixing his hernia last year, but since then he has had the old person's curse - pretty much incontinent - and appears to have dementia. Not to mention being not quite totally deaf and going blind. You will by now be asking why he is still with the living, and the answer is he has been with us so long, no one wants to take him to the vet. To look at him, he is in great shape and we are told he has the heart of a 10-year-old, so that is not going to carry him off. It looks like nothing natural is.
The reason I think he has dementia is because he stands for ages in one spot, as if trying to remember what he wanted to do. In a person, this is considered normal - we all do that from time to time, as a result of busy lives and too much to remember - but he only eats, poops and sleeps (22 hours out of every 24). He also wanders into corners and finds himself behind pot plants and suchlike. This causes him to woof mournfully for hours on end, and we need to constantly be around to ensure that he is taken inside or removed from the corner and given a biscuit to reassure him and stop the barking. It is enough to make us all demented, and I am sure the neighbours, too, but it is such a hard decision to end his life.
Oh dear.

Monty in his prime