The World Population Prospects: The 2017 Revision, published by the UN Department of Economic and Social Affairs, said that currently China with 1.41 billion inhabitants and India with 1.34 billion remain the two most populous countries, comprising 19 and 18 per cent of the total global population.
"In roughly seven years, or around 2024, the population of India is expected to surpass that of China," the report said.
The 2017 Revision of World Population Prospects is the 25th round of official UN population estimates and projections.
In its 24th round of estimates released in 2015, it was projected that the population of India will surpass that of China's by 2022.
The new estimates released here today said that in 2024, India and China are expected to have roughly a population of 1.44 billion each. After that, India's population is projected to continue growing for several decades to around 1.5 billion in 2030 and approaching 1.66 billion in 2050, while the population of China is projected to remain stable until the 2030s, after which it may begin a slow decline.
India's population may eventually see a decline in the half century after 2050 to 1.51 billion by 2100 but it will still be the most populous country in the world.
In India, life expectancy at birth will be 71 years in 2025-2030, growing to 74.2 years in 2045-2050. The under-five mortality rate will decline from 32.3 deaths under age five per 1,000 live births in 2025-2030 to 18.6 in 2045-2050.
The report said the current world population of 7.6 billion is expected to reach 8.6 billion in 2030, 9.8 billion in 2050 and 11.2 billion in 2100.
"With roughly 83 million people being added to the world's population every year, the upward trend in population size is expected to continue, even assuming that fertility levels will continue to decline," it said.
Ten countries are expected to account collectively for more than half of the world's projected population increase over the period 2017-2050: India, Nigeria, Congo, Pakistan, Ethiopia, Tanzania, the United States, Uganda, Indonesia and Egypt.
Among the ten largest countries worldwide, Nigeria is growing the most rapidly. Consequently, the population of Nigeria, currently the world's 7th largest, is projected to surpass that of the US and become the third largest country in the world shortly before 2050.
Further, large and persistent economic and demographic asymmetries between countries are likely to remain key drivers of international migration for the foreseeable future. Between 2015 and 2050, the top net receivers of international migrants (more than 100,000 annually) are projected to be the US, Germany, Canada, UK, Australia and Russia.
The countries projected to be net senders of more than 100,000 migrants annually include India, Bangladesh, China, Pakistan and Indonesia.
The report said that in 2010-2015, around 46 per cent of the world's population lived in intermediate-fertility countries, where women have on average between 2.1 and 5 births over a lifetime.
Intermediate-fertility countries are found in many regions, with the largest being India, Indonesia, Pakistan, Bangladesh, Mexico and the Philippines.
In 2045-2050, it is expected that slightly less than a third of the world's population will live in countries with fertility in this range. By that time, most of the world's population will be living in countries with relatively low levels of fertility, where women bear fewer than 2.1 children on average.
The report highlights that a reduction in the fertility level results not only in a slower pace of population growth but also in an older population. Compared to 2017, the number of persons aged 60 or above is expected to more than double by 2050 and to more than triple by 2100, rising from 962 million globally in 2017 to 2.1 billion in 2050 and 3.1 billion in 2100.
"Population ageing is projected to have a profound effect on societies, underscoring the fiscal and political pressures that the health care, old-age pension and social protection systems of many countries are likely to face in the coming decades," it said.
Substantial improvements in life expectancy have occurred in recent years. Globally, life expectancy at birth has risen from 65 years for men and 69 years for women in 2000-2005 to 69 years for men and 73 years for women in 2010- 2015.