I consider myself more tuned in to my pets than most pet parents. Our veterinarian was amazed at how quickly we noticed changes in Buddy. His renal failure was diagnosed in its earliest stages because we observed almost imperceptible differences in his behavior. Make no mistake about it; cats especially are very adept at hiding illness. It is an ingrained defense mechanism needed for survival in the wild.
Why did I miss all this? Actually, I didn't but without so much as a thought about it I just accepted the changes as part of aging or as depression brought on by the integration of Orphée into our household. That was my mistake - I did not question whether or not the symptoms were part of some illness or if they were from aging and could any steps be taken to minimize or eliminate them.
Lesson learned. Pay attention to the slightest change in behavior or temperament. Remember as your pet ages you must adjust your mindset to the needs of a four-legged senior. Your pet will be all the better for it.