When an old cat stops using the litter box, it's worth looking at the bigger picture — not just the behavior, but their overall condition. Lethargy, weight loss, increased thirst, more frequent urination, changes in appetite, or a strong smell to the urine can all point to a medical cause.
Senior cats are prone to kidney disease, diabetes, urinary tract infections, arthritis, and cognitive changes. Sometimes a cat simply forgets the route to the box, or starts feeling anxious in a space that used to feel completely familiar.
If your old cat is peeing or pooping outside the litter box repeatedly, rule out health issues first — then look at behavior.