My cat is heading towards 13 years old. She is still pretty active, can leap tall fences in a single bound etc, but does spend a lot of time sleeping, and is kept indoors overnight.
But she doesn't eat very much. We carried on feeding her the same biscuits as her previous owners (adopted a few months ago), split over 3 meals in fresh bowls, but there is always leftovers. She has multiple water sources including rainwater dishes outside.
So I have started adding in a sachet of wet food as well, which she gets very vocal about in advance. And she's hanging round us at mealtimes too, but that could just be naughty!
I've never had issues with a cat not eating properly before. Could it just be her age catching up with her - I have never had a cat this old before, so not sure what to expect. She was fully MOTd before I got her, all jabs up to date, flea'd and wormed etc and seems in otherwise perfect health and quite content with us.