What Is the Secret of a Long Life?
F or the first time in recorded history, the number of people aged over sixty-five in many developed nations now exceeds the number under eighteen. Within this ageing population, the fastest-growing demographic is not merely the elderly but the very old -- those who have survived past ninety or a hundred years. In the United Kingdom alone, the number of centenarians has increased more than sixfold since the beginning of the twentieth century, and projections suggest that by 2030 there will be approximately one million people aged a hundred or over worldwide. Understanding what separates these long-lived individuals from the majority of the population has become one of the most active fields in gerontological research.
The popular assumption -- that extreme old age is accompanied by prolonged disability and dependence -- appears to be largely mistaken. Studies of centenarians in multiple countries have consistently found that those who survive into their tenth and eleventh decades tend to have spent a greater proportion of their lives in good health than people who die at eighty or eighty-five. The oldest old, it seems, do not live longer by enduring a longer period of decline; they compress illness and disability into the final phase of life. Supercentenarians -- those aged a hundred and ten or above -- show this pattern most strikingly: many have functioned independently until the age of one hundred and five or beyond, several years longer than the typical centenarian.
A large-scale Danish study that followed every surviving person born in 1905 found that the proportion capable of living independently remained remarkably stable at approximately one third throughout the study period, as the least robust individuals died earlier and left a progressively more resilient group of survivors. Research in England and China reached broadly consistent conclusions: the majority of the very old are not living in institutional care, and many retain significant cognitive and physical capacity well into their final years.
Analysis of the medical histories of centenarians has revealed three broad patterns. Approximately forty percent are delayers -- individuals who avoid the onset of serious chronic disease until after the age of eighty. A further forty percent are survivors -- those who developed chronic conditions before eighty but outlived them. The remaining twenty percent are escapers -- individuals who reach a hundred with no evidence of the most common age-related diseases. Intriguingly, male centenarians are considerably more likely to fall into the escaper category than female ones, even though women make up the large majority of centenarians overall. Geographical clusters of exceptional longevity have attracted particular scientific attention. The Japanese island of Okinawa, which has the highest proportion of centenarians anywhere in the world, along with the Italian island of Sardinia and Iceland, shows rates of extreme longevity that far exceed national averages. All three are relatively isolated island communities, and researchers have proposed that reduced genetic variation among their populations has concentrated longevity-promoting genetic variants. Shared environmental and cultural factors -- including traditional diets, high levels of physical activity, and strong social networks -- complicate the picture, since these communities also differ from national averages in their lifestyles.
Twin studies from Scandinavia, in which the lifespans of identical and non-identical twins were compared, have suggested that genetic factors exert relatively little influence on longevity before the age of sixty, but become progressively more important thereafter. Social and environmental factors -- diet, exercise, wealth, education -- appear to dominate in earlier life, while genes increasingly determine who among the already-old survives to the most extreme ages. This finding helps explain why longevity tends to run in families without being predictable from family history alone in younger individuals.