International Institute for Population Sciences
Deemed to be University (An Autonomous Organization of Ministry of Health & Family Welfare, Government of India)

The mean age (life lived) of a stationary population is equal to the average remaining life expectancy (life left). The originator of this relationship, Lotka (1998), states that an individual aged a_bar is halfway through his/her life, and one can approximate the mean age by finding the age at which remaining life expectancy equals the age, i.e., e(a) = a. The halfway age, the age at which remaining life expectancy equals the age, is a robust measure for comparing mortality improvement (or survival gain, equivalently) over time across geographic areas and the stages of mortality transition. The halfway age metric is distribution sensitive as it takes into account the changing age distribution of death. Here, we explored the empirical trends of halfway age in 237 countries/areas categorised under five groups based on their stage of mortality transition. We computed the halfway age for 150 years from 1951-2100 for each country and compared their trends within and across countries in the stages of transition. Next, we computed the halfway age from the cohort life table for the 237 countries/ areas and compared it with the period measure. Finally, we compare the relative ageing index for each country over 1951-2100 and compare their convergence to the value 12, occurring in the stationary population. We found that, with time the life expectancy shows a convergence with the double halfway age. However, there are large differences across countries. High life expectancy countries, including Japan and the Republic of Korea, show convergence to stationarity. A wide distance from stationarity is observed in Afghanistan, Bangladesh and Pakistan.