Old age comes three years later for every generation
Half a century of data reveals population ageing can be described as a moving front. Nick Carne reports.
The age at which we die has been slowly moving forward like a wave, and there is “no evidence” of an impending limit to how long we can live, new research suggests.
Both findings are sure to fire up ongoing debates.
In the past two years alone, separate studies have concluded that the maximum lifespan of humans is fixed or that it isn’t, and – as the new research acknowledges – there is no clear agreement about the age pattern of old-age deaths.
Some argue that death numbers should become compressed at advanced ages, others that they should become more dispersed, and others still maintain that they are consistent, with little change in dispersion.
However, when researchers from Stanford University in the US and China’s Huazhong University of Science and Technology analysed annual life-tables from the Human Mortality Database (HMD) for the 50 years between 1960 and 2010 in 20 industrialised countries, they concluded that old-age survival follows an advancing front.
The long-term speed of the front is approximately 0.12 years per calendar year, or about three years per human generation. Thus, age 68 today is equivalent, in terms of mortality, to age 65 a generation ago.
The male survival front has lower long-term speed and is more dispersed than the female one. Front behaviour was similar over the five decades for females and males in all of the countries studied.
“Our unexpected result underscores the plasticity of old-age human mortality, with deaths steadily delayed as societies develop, and supports an ongoing increase in the age of transition to disability,” the researchers, led by Stanford’s Wenyun Zuo, write in a paper published in the journal Proceedings of the National Academy of Sciences.
That last point, though not expanded on in detail, is perhaps the most important, because it raises the question of what “old” means in a modern society, and when support for the elderly should kick in. As the paper notes, Japanese females had a roughly 80% probability of living past age 60 years of age in 1960, past 70 by 1977, and past 80 by 2011
The researchers note that their results suggest endowments, biological or other, are principal determinants of old-age survival.
“The advancing survival front that we find suggests that the effects of inequality on mortality may be much smaller among old-aged adults than among younger adults,” they write.
“Our analyses use period life tables, not cohorts, and suggest that continued mortality improvement depends largely on period processes such as economic growth, investment and advances in health science research and practice, and increases in the age of transition to disability.”
The researchers cannot explain differences in the location and speed of the advancing front between genders or countries, but suggest that the surprising regularity of their findings should be used to improve mortality forecasts.