There is an old country song by Tom T. Hall called Old Dogs and Children, and Watermelon Wine. I've never had watermelon wine (and, truth be told, I'm one of the very few people in the deep south who doesn't much care for watermelon at all). But I'm becoming something of an authority on old dogs.
Buddy, my loveable mutt, is over 13. Using the standard 7 people years to 1 dog year ratio, that puts him in his early 90's. I have a grandmother in Texas who is 91... she's in a nursing home. My step-grandfather is also 91... he's not exactly getting around quickly. But Buddy is older and slower than both of them. His front half is German shepherd, and his back half is golden retriever. The retriever half seems to have given out. Buddy has almost lost the use of his back legs. He pretty much lays in one spot most of the time, gets up VERY slowly to hobble to his food and water, and then goes back to his spot. His main activities are sleeping and accepting affection (mostly from Mary Elizabeth and me). When he goes outside, I have to carry him up and down the three steps out the back door (which is complicated by the fact that he weighs about 75 pounds).
We've had the vet in to look at him. She says he seems happy and comfortable, and that he'll let us know when the time comes. I certainly wouldn't want to euthanize Buddy just because he has become inconvenient. But I don't want him to have any discomfort, either. Regardless, Buddy is the best dog I've ever had. And I'm going to miss him when he's gone.