More people are living longer and are expected to continue living longer well into the 20th century, a team of 14 international scientists said Thursday.

"The major take is longevity is going up, more rapidly than we can explain," said Thomas Johnson, professor of behavioral genetics at the University of Colorado at Boulder, said."Humans can't live forever, but there is no real limit."

Johnson is one of the researchers from institutions in Europe, the United States and Mexico who found that 15 percent of the world population in 2025 will be 60 or older.

This figure is up from just nine percent in 1997, according to the study published in the journal Science.

Led by James Vaupel of Max Plank Institute for Demographic Research in Rostock, Germany, researchers studied the life expectancies of people in developed and developing nations by using a technique called biodemographics, a mix of biology, demography and statistics.

After gathering data from various countries, they found the countries projected to have the highest percent of people over 60 will be Japan and Italy, with 33 percent.

By 2025 32 percent of Germany's population will be 60 or older, they predicted, and in Sweden 29 percent of the population will be 60-plus, they predicted.

The percent of over-60s in the United States is expected to climb to 25 percent of the population from 17 percent in 1997, the scientists said.

In developing countries like India and Mexico, the elderly population is expected to increase over the quarter-century to at least 12 percent.

This is in part due to the aging of the baby-boom generation, better medical care and nutrition. But there is another factor, they said: Once people make it to old age, they hang in there - especially women.

"Since the early 1970s female death rates in Japan have declined at annual rates of about 3 percent for octogenarians (80-year-olds) and two percent for nonagenarians (90-year-olds)," the researchers wrote.

"Mortality among octogenarians and nonagenarians has been low in the United States." They also found that the population of 100-year-olds of various developed countries has doubled every 10 years since 1960.

This is perplexing, they said. Perhaps genetics were a factor, or acquired characteristics such as better nutrition.

But the trend does not hold just in people. Wasps, a nematode worm, baker's yeast and four species of fruit flies also tend to live longer as the species survive.

So the researchers suggest that some higher process - mathematics, for example, might be responsible. A deceleration in mortality could simply be a general property of complex systems.

There may be ways to speed up this process. Johnson, who worked on nematodes and roundworms, said that when a gene known as ag-1 is eliminated in nematodes, they live longer - almost twice as long.

"I was incredulous," he said. "I replicated 14 times." With ag-1, nematodes normally live about three weeks, Johnson said, but without it they lived five to six weeks.

It is easier to delete genes from worms than from people, but Johnson plans to try the trick on mice next.