Currently 60 percent of India's 1.2 billion people are under age 30, according to the United Nations.
Its working-age population, between 15 and 64, will grow nearly 18 percent by 2020, compared with only 0.3 percent growth for China, according to Euromonitor International.
But high levels of illiteracy and unemployment are eating into some of this demographic dividend.
"Asia as a whole is aging for sure, and this poses two big strategic challenges," said Donghyun Park, senior economist at the Asian Development Bank (ADB).
Singapore has the lowest fertility rate among 222 nations, according to the World Factbook, with a woman expected to give birth to an average 0.78 children during her lifetime.
In Japan, the rate of 1.39 has stayed around the same level for the past decade, while in China it is 1.55.
This is putting pressure on Beijing to rethink its one-child policy started in 1979.
Governments Get Into the Act Several Asian countries have come up with special programs and innovative ways of encouraging people to get married and have more kids.