Trajectories of Extreme Survival in Heterogeneous Populations

Recent gains in life expectancy among the elderly have noticeably contributed to increasing average life expectancy in developed countries. The old and oldest old are reaching thresholds that were unthinkable 30 or 40 years ago. Are recent gains due to increased longevity among a growing proportion of the population? Or are such gains the harbinger of new frontiers that may announce the further "extension" of the survival curve? Deeper comprehension of underlying mechanisms hinges on models that consider the impact of heterogeneity in individual frailty. In this paper, we analyse the mortality trajectories of French women born between 1820 and 1879. We applied a classic frailty model and a mixture frailty model accounting for individual differences both in the level of mortality and in the rate of ageing. The survival trajectories obtained with these models were used to estimate the maximum life span. Moreover, a non-parametric approach was applied to female centenarians born in France between 1870 and 1879 to estimate the extreme age at death. Results confirm that population heterogeneity can be an important factor in the dynamics of mortality at the oldest ages. In particular, the mixture fragility model may fit the data better and estimate the possible maximum life span more accurately. A tendency towards an increasing human life span clearly emerges, without however establishing whether a limit in fact exists