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.