The Process of Ageing 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 possibility 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 vague 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 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 physically strong 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 ran out of energy in accordance with the second law of thermodynamics  . But these are not similar or equivalent 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. What can be learned from this passage is that   _  .
people usually are unhappy when they are reminded of ageing children reach their full intelligence at the age of twelve years people are usually more likely to die at the age of twelve years our first twelve years represent the peak of human development