LONDON  - More than half of babies born in rich nations today will live to be 100 years old if current life expectancy trends continue, according to Danish researchers.

Increasing numbers of very old people could pose major challenges for health and social systems, but the research showed that may be mitigated by people not only living longer, but also staying healthier in their latter years.

"Very long lives are not the distant privilege of remote future generations -- very long lives are the probable destiny of most people alive now in developed countries," Kaare Christensen of the Danish Aging Research Center wrote on Friday in a study in the Lancet medical journal.

The study used Germany as a case study and showed that by 2050, its population will be substantially older and smaller than now -- a situation it said was now typical of rich nations.

This means smaller workforces in rich nations will have to shoulder an ever-greater burden of ballooning pension and healthcare requirements of the old.

Many governments in developed nations are already making moves toward raising the typical age of retirement to try to cope with aging populations.

The researchers said this was an important strategy, and added that if part-time work was considered for more of the workforce, that could have yet more benefits.

"If people in their 60s and early 70s worked much more than they do nowadays, then most people could work fewer hours per week," they wrote. "Preliminary evidence suggests that shortened working weeks over extended working lives might further contribute to increases in life expectancy and health."