My experience says a cat that age may indeed lose the fat on back and sides, muscle waste. It's easy to feel the backbone, and somehow such a cat feels more fragile to me (in my hands). It may be gravity, everything sags, and the looseness of the skin in old age (loss of good strong connective tissue). Am there myself, my own body. Pfffft.
That said, do be aware that there are things that can make a cat look and feel (to the touch) *old* - one is thyroid issues, another is kidney issues... Bad stuff can happen in old age, something has to be "the end". But as long as the cat is happy, eating, sleeping, pooping, mabye a bit of play - well, what are we to do?
I hope we can all age gracefully. And plan ahead for when the end times come.
My oldest cat currently is in his 15th year, and has "muscle waste" too and has no fat on his back or hip bones either, like he did in earlier years. He just had a vet checkup, blood work, etc. and is all OK. He's still active and strong, and his 4 y.o. cat friend Fitty Pisky keeps him on his toes, and the two often have races up and down the stairs.
Thank you all of you! Ben can't really play as he got too fat (vet never warned me) and his retina detached BUT each night I HAVE to play tug of war with him. He IS eating pretty good but does not care for the RX food.