Claims such as this about future life expectancy are based on the assumption that recent trends in increasing life expectancy continue. It is impossible to say for certain whether these trends will continue, and there is some evidence to suggest they won't.
The article Ageing populations: the challenges ahead published in The Lancet presents this forecast of life expectancy:
This forecast is based on the assumption that
mortality before age 50 years will remain at 2006 levels. At
age 50 years and older, probability of dying decreases by a
rate that yields yearly improvements in period life
expectancy of 0·2 years.
This response in The Lancet to the above article raises a number of objections:
Consider that published period life tables for Japanese women, the longest-lived population in the world, indicate that only 4·6% of the cohort born in 2000 are expected to live to 100 years. In the USA, this expectation is 2·0%. In other words, the 50% survival assertion by Christensen and colleagues is 11 times and 25 times greater than estimates for Japan or the USA, respectively, for the year 2000—implying that period mortality underestimates cohort life expectancy by decades.
Christensen and colleagues state that life expectancy is lengthening almost linearly in most developed countries, with no signs of deceleration. On the contrary, in England and Wales, Germany, the Netherlands, and the USA, life expectancy at age 65 years for women has converged to about 19·5 years, and has stagnated at that level in the USA and the Netherlands for most of the past 20 years.
Also, this article in The New England Journal of Medicine suggests that life expectancy may level off or even decline due to the effects of obesity.
From our analysis of the effect of obesity on longevity, we conclude that the steady rise in life expectancy during the past two centuries may soon come to an end.