My two cats are geriatric and I have had them since they were kittens. My Siamese is 12 years old, while my tabby is 16. The tabby cat had urinary crystals as a young cat and I had them both on Hill's C/D diet for many years. Then I went to wet food because it was healthier, but store bought brands because of convenience.
My Siamese is pretty healthy. I hadn't had her to the vet for more than checkups and rabies shots, until she got a double ear infection last December. She also regularly had a small amount of crusty discharge on one of her eyes. Someone suggested it could be a food allergy, and I felt the guilt for feeding her Fancy Feast and Friskies. Although I was trying to buy only the varieties my vet recommended as low carb, I knew they put corn and crap in there she didn't need.
So off to pet store I go with my reading glasses to check labels on all the cat food. I landed on Blue Freedom, grain free cat food for mature cats. Grain free is big for both cats and humans these days.
Perhaps with good reason, as it has only been a handful of months and I see a marked improvement in my cat that I didn't know was needed. The ear infection cleared up (with drops), but so did the eye discharge, her fur is silkier than before and even her stance is more regal and Siamese like. She has never been overweight, but her belly was round and now it is less so. It has to be the food! I am surprised at what a difference a high quality diet can do.
As for the tabby, he has another Hill's prescription diet for another health problem. He looks terrible, but it does seem to be addressing his issue. I wish I could put him on the Blue Freedom food as well, but that would lead us down a road not right for that cat.