This sounds like what one of mine started going through when he was 19. At the beginning of year 19 the vet said he was in excellent health especially for a cat that age. He had to be put down 8 months later. Everything caught up with him suddenly. He was skin and bones, dementia, going blind, going deaf, yowling at night. Near the end he started having seizures and stability issues. He was a sweet boy and a very cuddly guy sometimes. I hated having to put him down, but it was the right thing to do.