RIP Taffy :(

We have to put our cat, Taffy, to sleep today. :(  He started walking funny a few days ago, but then he seemed to be fine.  Then yesterday, he started going to bathroom all over himself so we took him to the vet this morning, and they said he's basically dying.  I really don't understand how a cat just starts dying in a day, but he is about 15 years old.  I feel really bad, and Ella is going to really miss him- she LOVES the cat.  She always tries to pet him and says "kitty" "meow".  He was a great cat!!