????退休后活得更長久的秘訣就是：首先得熬到退休。

????我們的平均死亡年齡在人生的最后1/3階段戶出現暴漲，人活得越久，就越可能活得更久。為什么我們的期望壽命會隨著年齡增加？就因為適者生存。“努力沿著年齡軸向上攀爬，沿途虛弱者基本上都從人群中清除出去了，”疾病預防與控制中心（Centers for Disease Control and Prevention，CDC）的死亡統計主管鮑勃安德森解釋道。“年齡越大時，人口中剩下的就越是最強者。隨著人的年齡遞增，這個趨勢一直貫穿始終。”

????死亡登記在美國是強制性的，CDC用各州生命記錄辦公室遞交的死亡證明數據來編制人口信息。這意味著，除了那些尸體后來才被發現的可憐人，我們幾乎知道所有給定的某年內死亡人口的年齡和死因。期望壽命通常是指：假定嬰兒在給定的某年出生，他的預期生存年數，這是基于死亡統計數據計算的。但在當年（下圖為2007年）存活的65歲老人的預期壽命會比任何更年輕的人要長。安德森說，那是因為老人一路上已經越過了很多致命的障礙。

????第一個死亡高峰發生在出生的頭一年，約有3萬嬰兒死于先天缺陷、早產和嬰兒猝死綜合癥（SIDS）這類問題。安德森說：“一旦度過了頭一年，人的生存幾率就提高了很多。”然后就是十幾二十歲的年紀，那是事故和暴力高發期。特別對男性，這是熱衷冒險的年齡，而20-24年齡段被稱為“事故高峰”。2007年，這個年齡段有超過1萬名男性死于事故或遇襲。（男性對風險的偏好從一個方面解釋了女性壽命更長的原因。雌激素對女性心臟的保護作用則是另一個原因。雌激素水平在絕經后下降，這也許能部分解釋為何期望壽命的性別差距從出生時的5年下降到100歲時的幾個月。）風險回避在25歲左右開始起作用，而自然死亡率開始上升。到45歲時，癌癥和心臟疾病、而不是事故，已經成為男性和女性的首要死亡原因。（順便提一句，自殺在45-49年齡段最為普遍。）一旦平安度過了脆弱的頭一年、熱衷冒險的青年階段和疾病纏身的中年階段，人們的平均壽命就會大幅上升。2007年出生的女性嬰兒預期壽命為80歲，而當年80歲的老太預期壽命高達89歲。

????在一個巨大而老齡化的人口中，長壽也有成本：按比例算，每個年輕工齡人口對應著更多的老人。“這也意味著，從人均來看，健康保健和社會保障將更為昂貴，”安德森說。“僅僅從人口學的角度說，很顯然相關費用會更高。”

????The key to living longer after retirement may simply be getting to retirement in the first place.

????Our average age at death soars in the last third of life, and the longer you live, the longer you're likely to live. Why does the total number of years we're expected to live increase with age? Survival of the fittest. "As you work your way through the age range, you're essentially weeding out the frail from the population," explains Bob Anderson, chief of mortality statistics at the Centers for Disease Control and Prevention. "When you get to the older ages, you're left with the more robust in the population. And that continues as you move up the age range."

????Death registration is mandatory in the U.S., and the CDC compiles demographic information from death certificate data filed by state vital records offices. That means we know the age of almost every person who dies in a given year -- minus a few people whose bodies are not found until later -- and why they died. Life expectancy is usually discussed as the number of years a hypothetical infant born in a certain year could be expected to live, and it's based on overall mortality statistics for that period. But the average age that, say, a 65-year-old alive in that year (2007 in our graph below) will reach will be better than that of anyone younger. That's because the older person has cleared more of life's deadly hurdles, Anderson says.

????The first hump happens in the first year of life, when around 30,000 babies succumb to problems like congenital defects, prematurity and SIDS. "Once you get past that first year, then your probability of survival is quite a bit higher," Anderson says. Then come the teens and twenties, when accidents and violence peak. For men, especially, these are the risk-taking years, and ages 20-24 are known as the "accident hump." In 2007, more than 10,000 men in this age group were killed in an accident or assault. (Men's tendency to take risks is one explanation suggested for why women live longer. The protective effect of estrogen on a woman's heart is another. Estrogen declines after menopause, which may be one factor in why the life expectancy gap between the sexes narrows from five years at birth to a couple months at age 100.) Risk-aversion begins to kick in around 25, and natural causes start to rank. By 45, cancer and heart disease are the leading killers of men and women, not accidents. (Suicides, incidentally, are also most common among 45- to 49-year-olds.) Once the frail first year, the risk-taking young-adult years and the disease-prone middle age have passed, the average age a person will achieve rises dramatically. Whereas a baby girl in 2007 could be expected to reach 80, an 80-year-old woman's expected age of death was 89 that year.

????Longevity in a large, aging population comes with a cost: proportionally more elderly people for every younger working person. "That means from a per capita standpoint, it's going be more expensive to maintain health care and social security," Anderson says. "Just from a demographic standpoint, it's clear to me that it makes things more expensive."