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So there is nothing to do for age. Human doctors define dying of old age as multiple organ dysfunction. Animals are so tolerant of high levels of age and disease that they just keep going along like nothing is wrong. They don't have the mental component of understanding aging like we do. She considers her age and discomfort as normal and just goes with it while a person would be effected mentally by being in the state Peggy is in.
So my advice is to talk to your vet about treating her arthritis and making her as comfortable as possible. From your description I find nothing in her daily activity that necessitates euthanizing her. The defecating thing could be an incontinence or unawareness from age but also the arthritis could be making her sore enough that she does not bother to hold it or posture to go regularly. I recommend an anti-inflammatory called Metacam for her along with a pain medication called Tramadol. I believe a combination of these two drugs will help her be more comfortable and get her to that 18th birthday. Thanks for your question
J Bache, DVM