LIVING BETTER?
Christensen and colleagues said huge increases in life expectancy -- of more than 30 years -- had been seen in most developed countries over the 20th century.
And death rates in nations with the longest life-expectancy, such as Japan, Sweden and Spain, suggest that, even if health conditions do not improve, three-quarters of babies will live to celebrate their 75th birthdays.
"But should life expectancy continue to improve at the same rate, most babies born in rich nations since 2000 can expect to live to 100 years," they wrote.
The researchers, who pooled and analysed data from several international studies, said they wanted to explore "a common view" that a big rise in the proportion of older people would come as a result of helping an increasing number of frail and ill people survive longer -- with huge personal and societal costs.
But they found that even though many people who live to age 85 have chronic diseases such as diabetes and arthritis, they have only become frail and disabled at a later stage, essentially postponing frail old age instead of extending it.
"This apparent contradiction is at least partly accounted for by early diagnosis, improved treatment, and amelioration of prevalent diseases so that they are less disabling," they wrote.
"People younger than 85 years are living longer and, on the whole, are able to manage their daily activities for longer."
But for people older than 85, the situation is less clear, the researchers said. Data are sparse, and there is widespread concern that exceptional longevity -- with ever larger numbers living to 100 and more -- could be grim for the people themselves and the societies they live in.