One of my favorite rebuttals to literalist Bible thumpers is that someone has already broken God’s upper limit of 120 – Jeanne Calment made it to 122. But everyone else is hitting the rev-limiter at 115. Will Oremus at Slate’s Future Tense investigates why:

In the past few years, the global count of supercentenarians—people 110 and older—has leveled off at about 80. And the maximum age hasn’t budged. Robert Young, senior gerontology consultant for the Guinness Book of World Records, says, “The more people are turning 110, the more people are dying at 110.”

Young calls this the “rectangularization of the mortality curve.” To illustrate it, he points to Japan, which in 1990 had 3,000 people aged 100 and over, with the oldest being 114. Twenty years later, Japan has an estimated 44,000 people over the age of 100—and the oldest is still 114. For reasons that aren’t entirely clear, Young says, the odds of a person dying in any given year between the ages of 110 and 113 appear to be about one in two. But by age 114, the chances jump to more like two in three.

It’s still possible that the barrier will eventually go the way of the four-minute mile. Steve Austad, a former lion tamer who is now a professor at the University of Texas Health Science Center, argues the apparent spike in mortality at age 114 is merely a statistical artifact. Today’s oldest humans, he’s reminds us, grew up without the benefit of 20th-century advances in nutrition and medicine. In 2000, he bet fellow gerontologist S. Jay Olshansky $500 million that someone born that year, somewhere in the world, would live to be 150. Olshansky, an Illinois at Chicago professor who wrote about the paradox of longevity for Slate last fall, doesn’t expect to be around in 2150 to collect his winnings. Even a cure for cancer or heart disease would do little to extend the maximum length of human life, he argues, because there are simply too many risk factors that pile up by the time a person is 115 years old. He believes supercentenarians owe their longevity more to freakish genes than perfect health; the 122-year-old Calment smoked cigarettes for 96 years. Olshansky and Austad agree on one point: A technological breakthrough, perhaps in the realm of genetics, that slows the aging process could send life spans surging upward.