Advancing front of old-age human survival

Zuo et al. examine the probabilities of death at ages past 65 years for males and females in developed countries; that is, they consider individuals in each year who are alive at age 65 y and thereafter experience death rates for that year. They conclude that an advancing old-age front characterizes old-age human survival in 20 developed countries. The long-term speed of the advancing front is ≃0.12 y per calendar year, about 3 y per human generation. Thus, the front implies that, e.g., age 68 y today is equivalent, in terms of mortality, to age 65 y a generation ago. Their finding of a shifting front in the percentiles of death at old age is consistent with some patterns of shifts in old-age mortality hazards.

Old-age mortality decline has driven recent increases in lifespans, but there is no agreement about trends in the age pattern of old-age deaths. Some argue that old-age deaths should become compressed at advanced ages, others argue that old-age deaths should become more dispersed with age, and yet others argue that old-age deaths are consistent with little change in dispersion. However, direct analysis of old-age deaths presents unusual challenges: Death rates at the oldest ages are always noisy, published life tables must assume an asymptotic age pattern of deaths, and the definition of “old-age” changes as lives lengthen. Here we use robust percentile-based methods to overcome some of these challenges and show, for five decades in 20 developed countries, that old-age survival follows an advancing front, like a traveling wave. The front lies between the 25th and 90th percentiles of old-age deaths, advancing with nearly constant long-term shape but annual fluctuations in speed. The existence of this front leads to several predictions that we verify, e.g., that advances in life expectancy at age 65 y are highly correlated with the advance of the 25th percentile, but not with distances between higher percentiles. Our unexpected result has implications for biological hypotheses about human aging and for future mortality change.