There might be no natural limit to how long humans can live—at least not one yet in sight—contrary to the claims of some demographers and biologists. That’s according to a statistical analysis published in Science on the survival probabilities of nearly 4,000 ‘super-elderly’ people in Italy, all aged 105 and older.

A team led by Sapienza University demographer Elisabetta Barbi and University of Roma Tre statistician Francesco Lagona, both based in Rome, found that the risk of death—which, throughout most of life, seems to increase as people age—levels off after age 105, creating a ‘mortality plateau’. At that point, the researchers say, the odds of someone dying from one birthday to the next are roughly 50:50.

If there is a mortality plateau, then it would seem that there is no limit to human longevity….That would mean that someone like Kane Tanaka, the Japanese lady who, at 115, is the world’s oldest known person, could live for years to come—or even forever, at least hypothetically.

Researchers have long debated whether humans have an upper age limit. The consensus holds that the risk of death steadily increases in adulthood, up to about age 80 or so. But there’s vehement disagreement about what happens as people enter their 90s and 100s.

Some scientists have examined demographic data and concluded that there is a fixed, natural ‘shelf-life’ for our species and that mortality rates keep increasing. Others have looked at the same data and concluded that the death risk flattens out in one’s ultra-golden years, and therefore that human lifespan does not have an upper threshold.

Age rage

In 2016, geneticist Jan Vijg and his colleagues at Albert Einstein College of Medicine in New York City rekindled the debate when they analysed the reported ages at death for the world’s oldest individuals over a half-century. They estimated that human longevity hit a ceiling at about 115 years—125 tops.

Vijg and his team argued that with few, if any, gains in maximum lifespan since the mid-1990s, human ageing had reached its natural limit. The longest known lifespan belongs to Jeanne Calment, a French super-centenarian who died in 1997 at age 122. (What tickles me enormously is that Jeanne apparently ‘lived long due to a heavy reliance on chocolate, olive oil, cigarettes and cheap red wine’). 

Experts challenged the statistical methods in the 2016 study, setting off a firestorm into which now step Barbi and Lagona. Working with colleagues at the Italian National Institute of Statistics, the researchers collected records on every Italian aged 105 years and older between 2009 and 2015—gathering certificates of death, birth and survival in an effort to minimise the chances of ‘age exaggeration’, a common problem among the oldest old.

They also tracked individual survival trajectories from one year to the next, rather than lump people into age intervals as previous studies that combine data sets have done. And by focusing just on Italy, which has one of the highest rates of centenarians per capita in the world, they avoided the issue of variation in data collection among different jurisdictions. As such, these data provide the best evidence to date of extreme-age mortality plateaus in humans.

Ken Wachter, a mathematical demographer at the University of California, Berkeley, and an author of the latest study, suspects that prior disputes over the patterns of late-life mortality have largely stemmed from bad records and statistics. Now that researchers have the advantage of better data, and if this same quality of data can be obtained from other countries, scientists expect to see a similar pattern elsewhere.

A French researcher, Jean-Marie Robine is not so sure. He says that unpublished data from France, Japan and Canada suggest that evidence for a mortality plateau is not as clear cut. A global analysis is still needed to determine whether the findings from Italy reflect a universal feature of human ageing.

Off limits

The world is home to around 500,000 people aged 100 and up—a number that’s predicted to nearly double with each coming decade. Even if the risk of late-life mortality remains constant at 50:50, the swelling global membership in the 100-plus club should translate into a creep upwards in the oldest person alive by about one year per decade, according to Joop de Beer, a longevity researcher at the Netherlands Interdisciplinary Demographic Institute in The Hague.

Many researchers say they hope to better understand what’s behind the levelling off of mortality rates in later life. Siegfried Hekimi, a geneticist at McGill University in Montreal, Canada, speculates that the body’s cells eventually reach a point where repair mechanisms can offset further damage to keep mortality rates level. Why this plateaus out and what it means about the process of ageing is still unclear.

For James Kirkland, a geriatrician at the Mayo Clinic in Rochester, Minnesota, the strong evidence for a mortality plateau points to the possibility of forestalling death at any age. Some experts think that the very frail are beyond repair. But if the odds of dying don’t increase over time, he says, interventions that slow ageing are likely to make a difference, even in the extremely old.

However not everyone buys that argument—or the conclusions of the latest paper.

Brandon Milholland, a co-author of the 2016 Nature paper, says that the evidence for a mortality plateau is “marginal”, as the study included fewer than 100 people who lived to 110 or beyond. Leonid Gavrilov, a longevity researcher at the University of Chicago in Illinois, notes that even small inaccuracies in the Italian longevity records could lead to a spurious conclusion.

Others say the conclusions of the study are biologically implausible: our body’s design imposes basic limitations, for example, cells that do not replicate, such as neurons (even though this is also debatable), will continue to wither and die as a person ages, placing upper boundaries on humans’ natural lifespan.

This study is thus unlikely to be the last word on the age-limit dispute but hopefully the debate – and the research surrounding it – will continue to have a very long (shelf) life.

References:

Barbi, E., Lagona, F., Marsili, M., Vaupel, J. W. & Wachter, K. W. Science 360, 1459–1461 (2018).

Dong, X., Milholland, B. & Vijg, J. Nature 538, 257–259 (2016).