The research, published Friday in the journal Science, suggests that any maximum fixed lifespan has yet to be reached -- and that human longevity is actually increasing.
"The increasing number of exceptionally long-lived people and the fact that their mortality beyond 105 is seen to be declining across cohorts -- lowering the mortality plateau or postponing the age when it appears -- strongly suggest that longevity is continuing to increase over time and that a limit, if any, has not been reached," the researchers wrote in the study.
Forging credible theories on the limits of human life demands solid data around mortality at extreme ages, according to the study. But given how few centenarians there are, good data have eluded researchers, said Joop de Beer, a population aging and longevity researcher at the Netherlands Interdisciplinary Demographic Institute in The Hague, who was not involved in the study.
As a consequence, the issue of whether human mortality limits are expanding has remained a contentious one.
"There's a big discussion about the human lifespan. Some experts say the probability of dying increases with age, even old age," de Beer said. "And other researchers say that the probability of dying levels off in old age and it would imply that people can live to very old ages. That there is no fixed limit."
This research "provides convincing evidence that the latter hypothesis is the true one. And it's consistent with what I've found," according to de Beer.
The new study has demonstrated that while the risk of dying increases as we age -- at 50, your chance of dying is three times higher than it was at 30 -- once we reach the age of 105, that risk plateaus at 50%, meaning you have a 50% chance of living another year.
"At this moment, the oldest person in the world, a Japanese woman, is 117. She has a 50% chance of living until she is 118. And again once she is 118, she has a 50% chance of becoming 119," de Beer said.
The study looked at 3,836 Italians 105 and older between 2009 and 2015. Inaccuracies resulting from poor data have hampered previous research. However, the quality of the data in this study allowed for a precise examination of extreme-age mortality, according to the research team.
Before the 19th century, average life expectancy fluctuated between 30 and 40 years
. However, tremendous advances in technology and medicine have extended life expectancies. Global life expectancy was 72 in 2016
Jeanne Calment, a French woman, still holds the record for oldest person
: She was 122 when she died in 1997.
The new research, while showing a plateau in the risk of death, also demonstrated a slight decline in mortality rates among people who reach 105 -- a trend that suggests human longevity might be increasing over time.
Yet more research with a larger sample is needed to win over some in the scientific community, according to de Beer. This could take a long time.
"The only thing we can do is wait until the number of centenarians increases, especially in Japan," de Beer said. "You would expect more people to reach the age of 110, but that will take some time. So I'm afraid that discussion cannot be decided in the next couple of years."