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Average remaining lifetimes can increase as human populations age


Increases in median ages, the most commonly used measure of population ageing1,2, are rapid in today's wealthier countries2,3, and population ageing is widely considered to be a significant challenge to the well-being of citizens there4. Conventional measures of age count years since birth; however, as lives lengthen, we need to think of age also in terms of years left until death or in proportion to the expanding lifespan. Here we propose a new measure of ageing: the median age of the population standardized for expected remaining years of life. We show, using historical data and forecasts for Germany, Japan and the United States, that although these populations will be growing older, as measured by their median ages, they will probably experience periods in which they grow younger, as measured by their standardized median ages. Furthermore, we provide forecasts for these countries of the old-age dependency ratio rescaled for increases in life expectancy at birth5. These ratios are forecasted to change much less than their unscaled counterparts, and also exhibit periods when the population is effectively growing younger.

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Figure 1: Unstandardized and standardized median ages, and life expectancies at unstandardized median ages.
Figure 2: Conventional and rescaled old-age dependency ratios.


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Correspondence to Warren C. Sanderson.

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Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Table S1

Data on unstandardized median age, life expectancy at the median age (median expected remaining years of life), standardized median age and standardized median age for subjects in study. (DOC 131 kb)

Supplementary Table S2

Conventional and Rescaled Old Age Dependency Ratios for Japan and Germany, 2000-2100. (DOC 53 kb)

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Sanderson, W., Scherbov, S. Average remaining lifetimes can increase as human populations age. Nature 435, 811–813 (2005).

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