My wife's cat that she had since she was a child was diagnosed with diabetes at 15. For the most part, it was fairly easy to manage. He lived for another 4 years. We did insulin injections twice a day when feeding him, and we fed him Hill's Science Diet W/D formula, per our vet's instruction.

Be sure to monitor your cat and watch for any changes in behavior. We had to adjust insulin dosages multiple times, usually slightly reducing the amount given.

Despite being older, once we was on insulin he went back to being his playful, albeit grumpy, self.

Unfortunately, we had to say goodbye to him in 2021 as he had developed advanced renal failure. It was very hard for my wife because she had him for all 19 years of his life and loved him dearly.

All that to say, diabetic cats can still have long happy lives as long as you are willing to provide it to them. I'm happy that my wife was able to spend a few more years with her best and longest friend.