长寿秘诀新解

    退休后活得更长久的秘诀就是：首先得熬到退休。

    我们的平均死亡年龄在人生的最后1/3阶段户出现暴涨，人活得越久，就越可能活得更久。为什么我们的期望寿命会随着年龄增加？就因为适者生存。“努力沿着年龄轴向上攀爬，沿途虚弱者基本上都从人群中清除出去了，”疾病预防与控制中心（Centers for Disease Control and Prevention，CDC）的死亡统计主管鲍勃安德森解释道。“年龄越大时，人口中剩下的就越是最强者。随着人的年龄递增，这个趋势一直贯穿始终。”

    死亡登记在美国是强制性的，CDC用各州生命记录办公室递交的死亡证明数据来编制人口信息。这意味着，除了那些尸体后来才被发现的可怜人，我们几乎知道所有给定的某年内死亡人口的年龄和死因。期望寿命通常是指：假定婴儿在给定的某年出生，他的预期生存年数，这是基于死亡统计数据计算的。但在当年（下图为2007年）存活的65岁老人的预期寿命会比任何更年轻的人要长。安德森说，那是因为老人一路上已经越过了很多致命的障碍。

    第一个死亡高峰发生在出生的头一年，约有3万婴儿死于先天缺陷、早产和婴儿猝死综合症（SIDS）这类问题。安德森说：“一旦度过了头一年，人的生存几率就提高了很多。”然后就是十几二十岁的年纪，那是事故和暴力高发期。特别对男性，这是热衷冒险的年龄，而20-24年龄段被称为“事故高峰”。2007年，这个年龄段有超过1万名男性死于事故或遇袭。（男性对风险的偏好从一个方面解释了女性寿命更长的原因。雌激素对女性心脏的保护作用则是另一个原因。雌激素水平在绝经后下降，这也许能部分解释为何期望寿命的性别差距从出生时的5年下降到100岁时的几个月。）风险回避在25岁左右开始起作用，而自然死亡率开始上升。到45岁时，癌症和心脏疾病、而不是事故，已经成为男性和女性的首要死亡原因。（顺便提一句，自杀在45-49年龄段最为普遍。）一旦平安度过了脆弱的头一年、热衷冒险的青年阶段和疾病缠身的中年阶段，人们的平均寿命就会大幅上升。2007年出生的女性婴儿预期寿命为80岁，而当年80岁的老太预期寿命高达89岁。

    在一个巨大而老龄化的人口中，长寿也有成本：按比例算，每个年轻工龄人口对应着更多的老人。“这也意味着，从人均来看，健康保健和社会保障将更为昂贵，”安德森说。“仅仅从人口学的角度说，很显然相关费用会更高。”

    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."