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.

　　人體在12歲時是生命力最旺盛的時期。雖然在這個時期人的身材、體力和智力還有待發展和完善，但在這個年齡死亡的可能性最小。再早一些，我們是幼兒和小孩子，身體較脆弱;再遲一些，我們就要經歷生命力和抵抗力逐步衰退的過程。雖然這個過程起初難以覺察，但最終會急轉直下，不管我們怎樣精心照料我們自己，不管社會和醫生怎樣對我們進行精心照顧，我們也無法再活下去了。生命力隨時間的流失而衰退叫做衰老。人類發現的最不愉快的一個事實是：人必然會衰老。既使我們能避開戰爭、意外的事故和各種疾病，我們最終也會“老死”;衰老的速度在人與人之間相差甚微，我們最可能死亡的年齡在65至80歲之間，有些人會死得早一些，少數人壽命會長一些——活到八十幾歲或九十幾歲，但這種可能性很小。不管我們多么幸運，多么健壯，我們所希望的長壽實際上是有限度的。

　　衰老的過程，不經提起，正常人容易忘記;一經提醒，才會記起。我們對人總是要衰老的現象并不陌生，多年來就已認識到。生命力隨著時間流失而喪失活力，人隨著年齡的增長而接近死亡，這是不言而喻的，就像一壺熱水遲早會涼下來，一雙鞋漸漸會磨破一樣。人們不但認識到所有的動物，大概也認識到所有的有機物，如樹木，甚至宇宙本身，從事物的本質上來說都會“磨損掉”。我們通常看到的大多數動物，即使能讓它們活得足夠長久的話，也會像我們一樣衰老的。像上緊發條的手表那樣的機械裝置，或太陽，也都會消耗完其能量(整個宇宙是否如此，目前尚有爭論)。不過，這些衰老的情況同人并不相似。手表停了依然是只手表，還可以重上好發條。然而一只老掉牙的手表，磨損太厲害，老得一點兒也不準了，最終會不值得修理了。但是，手表決不會自行修理，它不是由有生命的部件組成，而是由金屬組成，而金屬可以隨著磨擦而磨損殆盡。而我們人，在一定時間內是可以自行修復的，除了暴病而死或意外事故外，至少足以克服一切一般疾病和事故。在12歲至80歲之間，我們逐漸喪失這種能力。能使我們在12歲時病倒的疾病，到了80歲可能會使我們一蹶不振而進入墳墓。假如我們能保持12歲時的旺盛生命力，那么我們當中的一半人過700年才死去，剩下的一半人再過700年，才會又減少一半。