Could this mean my ageing cat is on her way out?

Animals have an extreme will to survive. From my past experience with either cats, dogs and also horses this in not a good sign. If she has teeth problems then this will stop her eating and would imagine your vet has already checked this out. I am sorry to hear that she is unwell and would possibly advise you to go back to the vet and have a chat as she maybe in pain if she has an underlying problem. If you feel she is suffering then you owe it to her to let her p away with dignity. You could spend lots of money and also suffering to her to locate the reason to why she is ill but you will end up with a hefty vets bill and a poorly cat. Please rely on your own judgement and decide what is best for her. Its a very sad time when you lose a pet that is part of the family and whom you are close too.