The awareness that we are growing old may be uniquely human. Perhaps other animals, particularly some of the higher mammals with rudimentary cognitive function, may be aware that they are slowing down in many ways, unable to keep up with others in their group, less able to find food or capture prey, slower to move away from danger or a predator. But we doubt they understand the full implications of the process. Humans do. Only we wonder how Lachesis measures out her thread; only we want to know the nature of Atropos's scisssors. Alone among the animal species that inhabit this planet, we are endowed with consciousness, an awareness of self—where we came from, and where we are going. We alone know, only too well, that the endpoint of aging is death. This drives a concern in us about the inner workings of the aging process—senescence—that is completely lacking in other species.
For some, old age is a time of peace and joy, of reflection on life's bountifulness, a time to contemplate the only immortality we can know: the lives of our children and grandchildren. With luck and a little attention to diet and exercise, we can maintain our ability to engage both physically and mentally in those lives, and in our community of friends. But it is not so for all of us. Genetics, accident, the “luck of the draw”—all of this can play against us in our later years, depriving us of the ability to participate fully and meaningfully in the world around us. All of us have this fear as we sense old age approaching, for it can happen to anyone. And at some level, even in the lucky ones, this fear never really goes away, because we know from observing other lives that at any moment our fate could change.