At the age of 12 years, the human body is at its most vigorous（强有力的（运动、活动）精力充沛的）. It has yet to reach its full size and strength, and its owner, his or her full intelligence. But at this age, the likelihood（可能性） of death is leased（出租）. Earlier, we were infants（婴儿） and young children, and consequently（因此）, more vulnerable（易受攻击的）. Later, we shall undergo（经历） a progressive（进步的） loss of our vigour（活力） and resistance, which, though imperceptible（觉察不到的） at first, will finally become so steep（陡峭的） that we can live no longer. However well we look after ourselves, and however well society and our doctors look after us.

  This decline（ 消减） in vigour（活力）, with the passing of time, is called aging. 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（使…相异） little from person to person, so that there are heavy odds（奇数） in favour of our dying between the ages of 65 and 80.