I think my oldest cat is getting senile. He's literally starving, as in lost half his body weight. Not because we aren't feeding him, or because of any digestive issues, or thyroid problems, or cancer or disease. We've had him checked. He's just not eating. We can get him to eat a little if we give him what he likes, like tuna, or chicken baby food, but he turns up his nose at everything else, even as he's begging for food.
On top of that, he's been pooping on the floor. He pees in his box, but poops on the floor.