Dear S.:
The litter box stuff sounds familiar. My cat was about the same age and NEVER had a litter box issue in his life, and suddenly he's going in all the other rooms. It turned out he had heart problems which were probably fueling some senility.
My first suggestion for your cat would be a trip to the vet; rule out anything medical. Your cat might have arthritis, but I'd also be afraid of a stroke.
My cat's litter box issues were all at night. We put in another litter box in our master bathroom and locked him in with us at night. Limiting his territory at night seemed to do the trick. He was my little boy, so being locked in with me away from our other cat suited him just fine.
Unfortunately, he died nine months later from heart failure. But getting him to the vet and on medication gave us time we wouldn't have had otherwise. Please, get your cat checked. I'm crossing my fingers it's nothing serious!
L. F., married 23 years, mom to a 14-year-old daughter, a 10-year-old cat and 5-year-old cat