My Mollie Rose, who will be 16 on Mother's day is VERY thin. Her bloodwork came back normal and the vet said it was due to age. I make her scrambled eggs with cheese and turkey every morning, and she noshes on that throughout the day. I also give her most of a can of cat food at night (splitting the remains between the other 4 cats). She's doing fine. She does drink alot of water and pees alot. But the vet didn't seem worried about it. If she stops eating or drinking water then I'll worry.