Raising the upper bounds of the human lifespan is turning out to be trickier than increasing the average person’s life expectancy. 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. Just seven people whose ages could be fully verified by the Gerontology Research Group have ever made it past 115.

The inventor Ray Kurzweil estimated in 2005 that, within 20 years, advances in medical technology would enable humans to extend their lifespans indefinitely. With six years gone and 14 to go, his prophecy doesn’t seem that much closer to coming true.

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 doesn’t expect to be around in 2150 to collect his winnings.