I think Peeples answer was great. Unfortunately we must decide when an animal no longer has quality of life and no longer enjoys the day. It is not just age, but if the dog is whining and no longer wants to be touched, like her dog, it is time.
I am attaching a photo of my handicapped goose, Football. She does not have the same life that the other geese have, but she is not suffering. Is her quality of life good? No. Is her quality of life acceptable? Yes, I cannot ask her but I think so.
But to do it without pain and guilt? I really do not think that is possible.
(my ISP connection is bad tonight and I cannot upload her photo)