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 soon,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
familier with the fact that man ages,that people have for years assued that the process of losing
vigour will 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 tun-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.