Because centenarians are so different from one another, in health history and other ways, it's impossible to predict which 60, 70, and 80-year-old individuals will reach age 100, according to Jeune.
In the United States, the 2000 census counted about one centenarian per 5,578 people. In 1990 the figure was one in 6,667. But that slight rise can be misleading, said S. Jay Olshansky, a biodemographer at University of Illinois at Chicago.
"We are unlikely to extend life expectancy by several decades within any of our lifetimes," Olshansky said. "That sort of quantum leap is not going to happen unless we figure out a way to modify aging."
Life expectancy from birth in developed nations is now at about 77—75 for men and 80 for women. Olshansky envisions a peak of 85 (82 for men, 88 for women) at least for the next 80 years or so.
Stretching the Lifespan of Worms, Flies, Mice—And Humans?
Raising life expectancy to 85 is an enormous challenge, Olshansky pointed out. "You would have to reduce all death rates, from all causes, for all ages, by more than 55 percent," he said.
Between 1900 and 2000, life expectancy jumped about 30 years—a rise associated with saving the young. "We have used up the easy gains," Olshansky said.
To date, a "calorie restriction" diet is the only method that consistently prolongs the lives of mice, rats, dogs, flies, spiders, nematodes, paramecium, and yeast. Now researchers are investigating whether the same approach extends life in primates—in particular, humans. But the diet requires a 30 percent calorie reduction from what is considered a healthy diet—too heavy a price for most.
In labs around the world, researchers have doubled and tripled the lifespan of worms—and increased the lifespan of flies and mice by between 30 and 80 percent—by manipulating the activity of certain genes and hormones. (Many of the genes are also found in humans.)
Last week, in a lab at the University of California, San Francisco, researchers stretched the lifespan of C. elegans, a microscopic worm, by six-fold.
Ordinary worms live about 20 days. The mutant worms live on average about 124 days. "What is really extraordinary is that the long-lived mutant worms suffered no loss of health or vitality," said Cynthia Kenyon, a molecular biologist and professor biochemistry and biophysics at UCSF. The study was published in the October 24 issue of Science.
The lab produced a movie of worms actively wriggling at 144 days. "In human terms," Kenyon wrote, "these animals would correspond to healthy, active 500-year-olds."
"I think one day it will be possible to affect hormone pathways in humans to increase lifespan, by possibly 10 or 20 years, but it is hard to say when that will be possible," Kenyon said. "The experiments in worms, flies and mice show us where to begin.
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