Surviving to be the oldest old—destiny or chance?

Abstract

Human beings have always been fascinated by super-longevity and now, as mortality declines to levels never before experienced, living to be a centenarian becomes a real, though still a rather remote, possibility. In most countries of the industrialised world the modal age at death is rising constantly and the number of centenarians is growing exponentially. A growing corpus of research is focussing on the centenarians, their particularities and the verification of their centenarian status. However, if we wish to unravel the secrets for attaining this magnificent age, we need to look beyond the centenarian communities themselves. The number of people becoming centenarians is determined by three different processes: the numbers born 100 years earlier, the probability of reaching old age and the probability of surviving old age to become a centenarian. Mortality up to age 80 and mortality beyond age 80, though loosely related, need to be treated as qualitatively different phenomena. However, there have been few studies of conditions distinguishing between populations and societies with low and high mortality at old ages. Instead, most work has focussed on identifying and carefully documenting high longevity populations, those with an unusually high number of centenarians -usually small, bounded populations -while no attention has been given to their complement, high shortivity populations, those with a lack of centenarians. In the absence of a theory of population longevity and shortivity, we cannot distinguish between populations which are genuinely different from most others, and those which are merely chance outliers. The critical question is thus 'what are the transition mechanisms'? We need to focus less on the centenarians themselves, and more on the conditions which distinguish populations with a relatively high probability of surviving from age 80 to age 100 from those with a relatively low probability of survival.