My little old girl was 20 when we had her put to sleep last year. She used to vomit often so we changed her to wet food. She'd lost a lot of teeth and didn't crunch biscuits any more. She used to call for us if we weren't in the room and got quite upset if she was on her own. She did lose a lot of weight near the end. We'd had her since she was 6 weeks old.
Right up until the end she would jump up on the bench to get her food no problems and as still very agile. Then one morning she jumped down well just never got up. I called the vet and made an app for a couple of hours later for her to be PTS. I felt that at 20 she was too old to be poked around when I'm sure her days were almost over.
It does sound like dementia and is pretty common in cats. Just take steps to keep her safe. Always being hungry...maybe check that out with the vets.