My cat Stubby is a little over 15 now. He still gets around real good. Has regular runnin fits and still lets the other cats know on a regular basis that he's still the alpha cat, without question. But every now and then he does have his bad days. Whether it's arthritis, we don't know. Doctor says it's tough to diagnose in cats, because they are active through the pain. So we give Stubby a small dose of ibuprofen when we see he's down and it helps.
