What's the limit of the human lifespan? And what do World War I veterans have to do with it?

Graph showing lower rate of mortality (blue) in people aged
90-95 versus the rate in people aged 50-55 (orange). Figure
from S.J. Newman (2018) Errors as a primarycause of late-life
mortality deceleration and plateaus. PLoS Biol 16(12):
e2006776. https://doi.org/10.1371/journal.pbio.2006776

An intriguing phenomenon has emerged in recent years: among very old people, the rate at which people die appears to decline when they get past a certain age. In other words, as these authors claimed in their 2011 book, aging slows down and maybe even stops. Or at least the mortality rate levels off past the age of 100, according to another study published earlier this year. This has led some scientists to speculate that the upper limit on human lifespan may be much older than anyone alive today.

Not so fast, says a new study by Saul Newman in PLoS Biology. Newman looked at the data and found something quite different: it's all just a mistake. Well, perhaps not a mistake exactly, but a consequence of many small errors. Let me explain.

In almost all species, mortality rates increase with age. In other words, as you get older, your likelihood of dying in a given year slowly but inexorably increases. Intuitively, we all know this: if young people die, it's tragic because we don't expect it. When people in their eighties and nineties die, it's sad, but no one is really surprised.

The evidence for decreasing mortality among very old humans has emerged from a number of studies that provide seemingly solid evidence that people over 100 die at the same or even lower rates then people between 80 and 90, or between 90 and 100.

Not surprisingly, many people would like to believe that human lifespan is unlimited. Indeed, it's one of the hottest topics in Silicon Valley these days. And perhaps someone will invent some true life-extension technology someday. But Newman's analysis pours cold water on the notion that our natural longevity is unlimited.

One difficulty with studying very old people is that there simply aren't that many of them, so the studies tend to be small. Another problem–and this is what Newman zeroes in on–is that we don't have very good birth records for people over 100 years old. They were born a long time ago, when record keeping wasn't always so good. What if there are a few errors?

It might seem that this shouldn't matter, as long as the errors are random–in other words, as long as people's ages are both under- and over-estimated at the same rates. The problem is that even if the errors are random, they don't play out that way. Here's why.

For the sake of argument, let's imagine a set of people whose true ages are off by 5 years in either direction. (I know that's a lot, but bear with me.) By the age of 100, as Newman points out, virtually no one is alive from the cohort that underestimated their age; these are people who have a true age of 105. But many more will be alive from those who overestimated their age; these are the 95-year-olds who think they're 100.

Newman's paper points out that if only a few people are overestimating their age, this can cause mortality rates to flatten or decelerate–or at least they appear to decelerate, because these people aren't really as old as we (or they) think they are. He then shows, in considerable detail, that only a very small error rate is more than enough to explain all of the apparent decline in mortality rates from recent studies. In other words, the decline in mortality is simply an illusion.

What does World War I have to do with any of this? Newman explains:

"approximately 250,000 youths inflated their ages to enter the 1894–1902 birth cohorts and fight for the United Kingdom in World War I."

The same thing happened in the U.S. and other countries: 16- and 17-year-old boys said they were 18 so they could sign up. Coincidentally, these men would have been around 100 years old when many of the recent studies of centenarians were conducted, and it's very likely that some of these men were included in those studies. It wouldn't take many to distort the apparent mortality rates.

Who could have imagined that these brave young men who signed up to fight for their country (my grandfather was one of them), so many years ago, would have this completely unexpected effect on the science of aging, almost exactly 100 years after the war ended? It seems somehow appropriate that today, as the last veterans of the Great War leave us forever, they can still remind us of their sacrifice.