India will remain a younger country than China in 2040, with a median age of 35 years compared to 44 years in China. China's population is expected to age faster over the next three decades, with the percentage of 60-plus people in total population projected to increase from 12.3 percent in 2010 to 27.5 percent in 2040. The corresponding increase in India is expected to be from 7.5 percent in 2010 to 15.6 percent in 2040. The difference in ageing will influence the proportion of the working age population to the total population in each country.

At present, the proportions of working-age people in the China's and India's total populations are 67.8 percent and 61.7 percent. China has the "demographic dividend" of having a larger workforce today. Come 2040, the proportions are expected to reverse, to 62.4 percent for China and 64.6 percent for India.

One needs to be cautious in claiming that the change will give a decisive economic edge to the Indian economy vis--vis China. In absolute terms, the proportions imply that in 2040 there will be about 1 billion working-age people in India compared with 0.9 billion in China. Given the sizes of the two economies, the difference is marginal. But given that India will continue to experience net additions to its population well after 2040, its working age population is expected to increase further. Hence, India's expected higher returns from the demographic dividend vis--vis China are likely to grow over a three-decade-plus time horizon from now.

These evolving demographics bring with them a complication for India, usually overlooked in the euphoria over the demographic dividend. The density of India's population (people per square kilometer) is projected to increase from 369 to 476 from 2010 to 2040. During the same period, China's population density will rise from 141 to 152. India will have to handle the challenge of accommodating its population growing at a faster rate than China's within a land area that is much smaller than China's.

The obvious implications of an adverse land-per-head ratio are greater pressures on natural resources and public goods. Inadequate supply responses on these fronts can easily erode much of the demographic dividend.

The author is head (development and programmes) and visiting senior research fellow at the Institute of South Asian Studies in the National University of Singapore.