在十二岁时，人体处于其最旺盛的状态。
At the age of twelve years, the human body is at its most vigorous.
此时身体尚未发育至完全成熟的体型与力量，其主人也尚未达到完整的智识水平；但在这一年龄段，死亡的概率是最低的。
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.
在更早的时候，我们是婴幼儿，因而更为脆弱；而在此之后，我们将经历一种渐进性的活力与抵抗力的丧失——这种丧失虽起初难以察觉，但最终将变得如此剧烈，以至于无论我们如何保养自身、社会和医生如何悉心照料，我们都将不再能够存活。
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.
这种随时间推移而出现的活力衰退，被称为衰老。
This decline in vigour with the passing of time is called ageing.
我们所有人都会作出的最令人不快的发现之一，就是：我们必然以这种方式衰退；如果我们能逃过战争、意外和疾病，终将"老死"；而这一过程发生的速率因人而异的差别极小，以至于我们在65岁至80岁之间死亡的概率是极高的。
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 65 and 80.
我们之中有些人会死得早一些，少数人会活得更长——一直活入第九个或第十个十年（即八九十岁乃至九十多岁）。
Some of us will die sooner, a few will live longer -- on into a ninth or tenth decade.
但概率是对我们不利的，而且我们所能期望的存活时长存在一个实际上的上限，无论我们多么幸运、多么强健。
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.
普通人往往会忘记这一过程，除非——确切地说，直到——被人提醒。
Normal people tend to forget this process unless and until they are reminded of it.
我们对"人会衰老"这一事实如此习以为常，以至于人们多年来一直想当然地认为，随时间推移而失去活力、随年龄增长而变得更容易死亡的过程，是像热水壶冷却或鞋子磨损那样不言自明的事情。
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.
人们还一直假定，所有动物，乃至可能还有树木之类的其他有机体，乃至宇宙本身，在事物的本性上都必然会"磨损耗尽"。
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'.
我们通常观察到的大多数动物，如果有机会活得足够长，实际上确实会像我们一样衰老；而像上好发条的手表或太阳这样的机械系统，实际上确实会依照热力学第二定律而耗尽能量（整个宇宙是否如此，目前仍是一个存疑的问题）。
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).
但这些与人类衰老时所发生的情况并不类似。
But these are not analogous to what happens when man ages.
一只走慢了的（耗尽发条的）手表仍然是手表，可以重新上发条。
A run-down watch is still a watch and can be rewound.
而一只老旧的手表，相比之下，则会变得如此磨损老化、如此不可靠，以至于最终不值得修理。
An old watch, by contrast, becomes so worn and unreliable that it eventually is not worth mending.
但手表永远无法自我修复——它不是由有生命的部件构成的，只由金属构成，而金属会因摩擦而逐渐磨损消耗。
But a watch could never repair itself--it does not consist of living parts, only of metal, which wears away by friction.
我们曾经能够修复自身——至少修复得足够好，以至于能够克服除最直接致命的疾病和事故之外的一切。
We could, at one time, repair ourselves--well enough, at least, to overcome all but the most instantly fatal illnesses and accidents.
在十二岁到八十岁之间，我们逐渐丧失这种（自我修复的）能力；一种在12岁时只会把我们击倒的疾病，到了80岁时却能将我们击毙，送入墓穴。
Between twelve and eighty years we gradually lose this power; an illness which at 12 would knock us over, at 80 can knock us out, and into our grave.
如果我们能够一直保持十二岁时的旺盛活力，大约需要700年才能有一半的人死去，再过700年，幸存者才会再减少一半。
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.