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. Earlier, we were infants and young children, and consequently
more vulnerable; later, we shall undergo a progressive loss of our vigour 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 vigour with the passing of time is called ageing. It is one of the
most unpleasant discoveries which we all make that we must decline in this way, that if
we escape wars, accidents and disease 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 favour 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. We
are so familiar with the fact that man ages, that people have for years assumed that the
process of losing vigour with time, of becoming more likely to die the older we get, was
something self-evident, like the cooling of a hot kettle or the wearing-out of a pair of shoes.
They have 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 chance 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 (whether the whole universe does so is a moot point at
present). 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. 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 this power; an
illness which at twelve would knock us over, at eighty can knock us out, and another 700 for
the survivors to be reduced by half again.