You need to let the vet decide. There could be other reasons for the problems with the back legs. That is pretty old for a cat. You have my sincere sympathy.
Yes, the sad truth is that it is his/her last days. I am sure you gave your cat good life and lots of love.
Other people asked questions on similar topics, check out the answers they received:
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them