New York ? The world’s population will age rapidly over the next 45 years in a signal of potential challenges for social welfare systems, according to a recent report released by the Population Division of the U.N. Department of Economic and Social Affairs.

By 2050, almost one in three people in the developed world will be older than 60, and the number of those older than 80 is expected to increase to 394 million in 2050, from the current 86 million.

The proportion of elderly in the developing world also is on the rise.

But by 2050, the populations of developed countries — Japan, Australia, New Zealand, and countries in Europe and North America — will decline annually by about 1 million people, while the populations of developing countries will rise by 35 million people annually, 22 million of whom will reside in the least developed countries of the world. With the least developed nations struggling to fight extreme poverty, disease, and starvation, such increases in population portend an increased number of crises.

More than 15 percent of Japan’s population will be aged more than 80 by 2050, according to the report. Japan has the world’s highest life expectancy at 81.9 years, which is projected to increase to 88.3 years by 2045-50, according to the report, “World Population Prospects: The 2004 Revision.”

The report said the world’s population, now 6.5 billion, would reach 9.1 billion in 2050, with almost all growth occurring in developing countries.

During the next 45 years, eight countries will account for more than half of the global population increase: India, Pakistan, Nigeria, the Democratic Republic of Congo, Bangladesh, Uganda, the United States, Ethiopia and China. By 2050, India’s population will surpass China’s, with the two countries combining to constitute half the world’s population.

The 44 developed nations are expected to witness a slight increase in fertility rates from 1.56 children per woman to 1.84 in 2045-50. However, with the exception of Albania, all developed countries show below-replacement fertility rates, meaning births do not offset the number of deaths.

As fertility rates decline, the global population ages. Between now and 2050, the number of people age 60 and older is expected to triple to 1.9 billion. The developed world is witnessing an aging of society, with the number of people over 60 already higher than the number of children 14 and under. By 2050, there will be two elderly people for every child in the developed world.

The Population Division’s report also examines the percentage of the working age population, age 15-59, as this figure has important implications for national and regional economies. The percentage of working age people in developed countries is at 63 percent in 2005, but is expected to decline to 52 percent by 2050.