My line is when they can't control their natural functions - but if your dog has lost its mind... well.
The loss of natural function has been my previous line in the sand.
Senility He looks good initially but he is functionally impaired.
This dog was a farm dog that ranged over 30 acres.
He is now tied to a lead, and shiver shakes and pants.
He cannot find me in the house.
He cannot figure out where the back door is.
I think it is tragic.
But I even had a neighbor who kept a blind dog alive for many years.
We had a cat who was often incontinent and my wife wanted to put her down. I objected on the grounds that it would establish a precedent. I'm probably just being paranoid..?