India is expected to surpass China and become the world's most populous country within the next three months, according to a recent United Nations Population Division report, marking a seismic shift on the world stage in trends with significant social and economic implications for both countries.

Of India's fast-growing 1.41 billion people, about one in four is under 15 and nearly half are under 25. By comparison, China's population is about 1.45 billion, but people under 25 make up only a quarter of the population.

Since 1950, India and China have accounted for about 35% of the world's population growth, with China becoming a global industrial power. Together, the two epicenters of population account for a significant portion of the world's roughly 8 billion people.

But China's one-child policy, introduced in 1980, has drastically reduced fertility rates and changed economic prospects. In recent years, women have been allowed to have up to three children, but the average fertility rate is still 1.2. China's population will peak in the coming years and is projected to decline.