Our old lady is going on 23. Physically she's not in bad shape. She's got a good appetite and good teeth to go with it. Her organs are holding up fairly well - she's drinking a bit more than she used to but not excessively and her weight is stable. She's getting a bit stiff and doddery but doesn't appear to be in any pain. She can still run up and down stairs and still goes out occasionally. She seems happy - purrs and chats a lot and loves a cuddle.
However I think there might be issues with dignity. She's become very unreliable with using the tray. She usually goes near it, sometimes she then gets in to try to bury it. She doesn't wash herself properly any more. Partly this is because she is stiff but mostly she's just forgotten I think. We spend a lot of time brushing her and rubbing her down with those 'grubby pet' wipes but, to put it bluntly, she smells.
The last week or so she has taken to climbing into the cat tray and curling up to sleep. This is what is making me feel that the time might have come to say goodbye. I don't think she has much of a clue what's going on, she is literally curling up and going to sleep on her own poo 
I don't think she's suffering but she always used to be such a dignified, clean cat and I'm finding this latest behaviour really upsetting. It's not something I'd wish for my own old age, even if I was too senile to be bothered.
DH thinks I'm anthropomorphising and that as she still seems happy and not in pain we should carry on. We all love her to bits and just want to do right by her so I'd be grateful for others' opinions.
Sorry to post yet another 'is it time?' thread - I can see there are a few on this topic at the moment. 
for anyone else who's facing a difficult decision.