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
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
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 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.&