['My old cat is very sick.', "I'm sorry to hear that.  How old is the cat?", "She is 23. If she isn't better in a few days, we will have to put her down."]
Ohh no.  That is a long life for a cat, and I know that makes it even harder.  I hope she gets better!
My old cat is very sick. We will have to put her down next week if she is not better.