At the age of twelve years, the human body is at its most vigorous. It has yet to reach its full size and strength, and its owner his or her full intelligence; but at this age the likelihood of death is least. 71. Earlier, we were infants and young children, and consequently more vulnerable; later, we shall undergo a progressive loss of our vigor and resistance which, though imperceptible at first, will finally become so steep that we can live no longer, however well we look after ourselves, and however well society, and our doctors, look after us. This decline in vigor with the passing of time is called aging. It is one the most unpleasant discoveries which we make that we must decline in this way, that if we escape wars, accidents and diseases we shall eventually "die of old age", and that this happens at a rate which differs little from person to person, so that there are heavy odds in favor of our dying between the ages of sixty-five and eighty. Some of us will die sooner, a few will live longer on into a ninth or tenth decade. But the chances are against it, and there is a virtual limit on how long we can hope to remain alive, however lucky and robust we are.
Normal people tend to forget this process unless and until they are reminded of it. 72. We are so familiar with the fact that man ages, that people have for years assumed that the process of losing vigor with time, of becoming more likely to die the older we get was something self-evident, like the cooling of a kettle of hot water or the wearing-out of a pair of shoes. They are also assumed that all animals, and probably other organisms such as trees, or even the universe itself, must in the nature of things "wear out". Most animals we commonly observe do in fact age as we do if given the chances to live long enough; and mechanical systems like a wound watch, or the sun, do in fact run out of energy in accordance with the second law of thermodynamics. But these are not analogous to what happens when man ages. A run-down watch is still a watch and can be rewound. An old watch, by contrast, becomes so worn and unreliable that it eventually is not worth mending. But a watch could never repair itself—it does not consist of living parts, only of metal, which wears away by friction. 73. We could, at one time, repair ourselves—well enough, at least, to overcome all but the most instantly fatal illnesses and accidents. Between twelve and eighty years we gradually lose the power; an illness which at twelve would knock us over, at eighty years can knock us out, and into our grave. If we could stay as vigorous as we are at twelve, it would take about 700 years for half of us to die, and another 700 for the survivors to be reduced by half again.