Editing Life expectancy (section)

==Life expectancy vs. other measures of longevity==
[[File:20200101 Remaining life expectancy - US.svg|thumb|"Remaining" life expectancy—expected number of remaining years of life as a function of ''current age''—is used in [[Pension|retirement income]] planning.<ref name="SocSecPeriodLifeExpectancy_2020">{{cite web|date=2020|title=Actuarial Life Table|url=https://www.ssa.gov/oact/STATS/table4c6.html|url-status=live|archive-url=https://web.archive.org/web/20230708231105/https://www.ssa.gov/oact/STATS/table4c6.html|archive-date=8 July 2023|publisher=U.S. Social Security Administration Office of Chief Actuary}}</ref>]]
Life expectancy may be confused with the average age an adult could expect to live, creating the misunderstanding that an adult's lifespan would be unlikely to exceed their life expectancy at birth. This is not the case, as life expectancy is an average of the lifespans of all individuals, including those who die before adulthood. One may compare the life expectancy of the period after childhood to estimate also the life expectancy of an adult.<ref name="Wanjek 2003"/>

As a measure of the years of life remaining, life expectancy decreases with age after initially rising in early childhood, but the average age to which a person is likely to live increases as they survive to successive higher ages.<ref>{{cite web|url=http://www.infoplease.com/ipa/A0005140.html|title=Life Expectancy by Age, 1850–2011|website=InfoPlease}}</ref> In the table above, the estimated modern hunter-gatherer average expectation of life at birth of 33 years (often considered an upper-bound for Paleolithic populations) equates to a life expectancy at 15 of 39 years, so that those surviving to age 15 will on average die at 54.

In England in the 13th–19th centuries with life expectancy at birth rising from perhaps 25 years to over 40, expectation of life at age 30 has been estimated at 20–30 years,<ref>{{cite web|vauthors=Khan-ad-Din FM, Caidan Pentathlon M|title=Old Age, Height and Nutrition: Common Misconceptions About Medieval England|url=http://sirguillaume.com/wp-content/uploads/2012/01/Old_Age-Height-Nutrition.pdf|access-date=28 January 2021|archive-date=7 February 2021|archive-url=https://web.archive.org/web/20210207054819/http://sirguillaume.com/wp-content/uploads/2012/01/Old_Age-Height-Nutrition.pdf|url-status=dead}}</ref> giving an average age at death of about 50–60 for those (a minority at the start of the period but two-thirds at its end) surviving beyond their twenties.

[[File:20200101 Life expectancy increases with age already achieved - chart.svg|thumb|Life expectancy<ref name="SocSecPeriodLifeExpectancy_2020" /> increases with age already achieved.]]

The table above gives the life expectancy at birth among 13th-century English nobles as 30–33, but having surviving to the age of 21, a male member of the English aristocracy could expect to live:
* 1200–1300: to age 64
* 1300–1400: to age 45 (because of the [[bubonic plague]])
* 1400–1500: to age 69
* 1500–1550: to age 71<ref name="Expectations of Life" />

A further concept is that of modal age at death, the single age when deaths among a population are more numerous than at any other age. In all pre-modern societies the most common age at death is the first year of life: it is only as infant mortality falls below around 33–34 per thousand (roughly a tenth of estimated ancient and medieval levels) that deaths in a later year of life (usually around age 80) become more numerous. While the most common age of death in adulthood among modern hunter-gatherers (often taken as a guide to the likely most favourable Paleolithic demographic experience) is estimated to average 72 years,<ref name="Gurven, Kaplan 2007">{{citation|vauthors=Gurven M, Kaplan H|title=Longevity Among Hunter- Gatherers: A Cross-Cultural Examination|journal=Population and Development Review|volume=33|issue=2|pages=321–365|year=2007|doi=10.1111/j.1728-4457.2007.00171.x}}</ref> the number dying at that age is dwarfed by those (over a fifth of all infants) dying in the first year of life, and only around a quarter usually survive to the higher age.

[[Maximum life span]] is an individual-specific concept, and therefore is an upper bound rather than an average.<ref name="Wanjek 2003">{{citation|url=https://books.google.com/books?id=oIJ5TKh7mPgC&q=%22this+is+one+of+the+biggest+misconceptions+about+old+age%22&pg=PA70|author-link=Christopher Wanjek|vauthors=Wanjek C|title=Bad Medicine: Misconceptions and Misuses Revealed, from Distance Healing to Vitamin O|publisher=Wiley|year=2002|pages=70–71|isbn=978-0-471-43499-3|postscript=.}}</ref> Science author [[Christopher Wanjek]] writes, "[H]as the human race increased its life span? Not at all. This is one of the biggest misconceptions about old age: we are not living any longer." The maximum life span, or oldest age a human can live, may be constant.<ref name="Wanjek 2003" /> Further, there are many examples of people living significantly longer than the average life expectancy of their time period, such as [[Socrates]] (71), [[Saint Anthony the Great]] (105), [[Michelangelo]] (88), and [[John Adams]] (90).<ref name="Wanjek 2003" />

However, anthropologist [[John D. Hawks]] criticizes the popular conflation of life span (life expectancy) and [[maximum life span]] when popular science writers falsely imply that the average adult human does not live longer than their ancestors. He writes, "[a]ge-specific mortality rates have declined across the adult lifespan. A smaller fraction of adults die at 20, at 30, at 40, at 50, and so on across the lifespan. As a result, we live longer on average... In every way we can measure, human lifespans are longer today than in the immediate past, and longer today than they were 2000 years ago... age-specific mortality rates in adults really have reduced substantially."<ref name="Hawks 2009">{{citation|url=http://johnhawks.net/weblog/reviews/life_history/age-specific-mortality-lifespan-bad-science-2009.html|author-link=John D. Hawks|vauthors=Hawks J|title=Human lifespans have not been constant for the last 2000 years|year=2009|postscript=.}}</ref>