At 18 he's probably suffering from Old Age. We had a Domestic Longhair that lived to about that age. She got to the point that she had trouble controlling her bodily functions, I.E., would actually poop in her bed while asleep, stopped grooming herself, couldn't jump, lost her hearing, and not sure about her eye sight. So no matter how hard it was we decided the best thing for her was to put her down. It's a tough thing to do, but there is such a thing as Quality of Life for our Fur Kids Too.