A baby born today in Japan can expect to live to 82 and enjoy the vast majority of those years in good health. Most people in Sierre Leone, however, probably will not see their 35th birthday.

Such inequality is blamed on poverty, high rates of AIDS infections and low levels of education.

A recent conference on aging held in Australia highlighted the gulf between life expectancy in rich countries and in the developing world. There are stark contrasts between nations in the same region. Australians, for example, will - on average - reach 80, while just to the north their neighbors in Papua New Guinea cannot expect to live past 60.

The differences in life expectancy between Canada and Haiti are even more stark: 79 years compared with just 50. People living in rich countries are not only living longer, they also are staying healthy longer. There is a new warning that the widening gap in health and life expectancy will ultimately cause massive social upheaval.

Alex Kalache from the World Health Organization says if these disparities continue, it could spark a mass movement of the world's poor, seeking better lives elsewhere. "This is just going to drive Africans completely crazy and then you cannot blame them if they will make the effort to leave their countries. The fear of this mass migration to Europe and other countries, Latin Americans to North America, Filipinos to Australasia or Indonesians," he says. "This is going to be a major headache."

Experts say aging populations will become big challenges for both the developed and the developing world in the coming years. Although poorer nations cannot match the life expectancies seen in wealthy countries, their citizens are living longer. That creates what has been described as "additional nightmares" for health services and economic resources that are ill equipped to handle rising numbers of frail older people. For modern, wealthy countries, other researchers believe aging populations will present not only problems but also opportunities.

The organizer of this inaugural longevity conference, Noah Weller, says societies can benefit from having more active and healthy older citizens. "It is an important issue that everyone of any age needs to look at," he says. "The key is to respect the elderly, be inspired by the wisdom and the joy that can be achieved by spending time with them."

The conference participants heard that the world is nowhere near ready for the anticipated explosion in the number of older people. In developed countries the elderly tend not to work and need more health care than the young, which can stress community resources.

Professor Bruce Carns of the University of Oklahoma in the United States thinks the aim should be a healthy life and not an unnecessarily long one. "The average age that a person lives today - a life expectancy at birth - of around 80, say, in some of the developed countries is far beyond the years needed to reproduce," says Mr. Carns.. "We have now bodies that are wearing out, and this costs a lot of money and so the goal of science, I think, should not be to extend life, it should be to improve the quality of life for the years we have."

There is some good news, however, for those who dream of immortality. A U.S. scientist claims he may have found the key to curing old age. Professor Michael Fossell from Michigan State University has said the body's biological clock can be re-set, making old cells behave like new. He is confident that humans could easily live beyond their 200th birthdays.