While the graying population around the world is rising in countries like Japan and South Korea, along with much of the United States and Europe, other parts of the world — primarily Africa — are experiencing the opposite situation, with young people claiming the majority.

The contrast between oldest and youngest populations can be stark: Japan has the world’s largest oldest population, with a mere 13 percent of its citizens under age 15, while 50 percent of Niger’s population is under 14 years old, making it the youngest nation globally in that age category.

The search for the world’s youngest populations and an analysis of the most recent population forecast from the United Nations leads to Africa, where the continent has more people under 20 years old than anywhere else in the world. The population is projected to double to two billion by 2050.

A closer examination narrows the youngest populations to sub-Saharan Africa, where the average percentage of the population under age 25 is 67 percent. Eight of the world’s top-10 youngest countries are located in this region, just south of the Sahara Desert. The remaining two countries, Timor-Leste and Afghanistan, are located in Asia.

The top-10 youngest countries, as of 2012:

Demographers say that the main cause of a young population is a consistent pattern of high fertility, which does not appear to be changing dramatically too soon. In sub-Saharan Africa, the average woman has five or more children; for example, Niger, the fourth-youngest country, averages 6.89 children per woman; Mali, another “young” nation, averages 6.1 children per woman.

Sangeetha Madhavan, the associate director of the Maryland Population Research Center at the University of Maryland, suggested that high fertility was compelled by high child mortality rates and “low levels of education for women, women’s lack of control over their reproduction and insufficient access to effective family planning.”

That said, while infant and child mortality (5 years old and under) remain higher in sub-Saharan countries (one in nine children dies before age 5), compared with the rest of the world, the youthful populations in the region are also driven by a steady decline in infant and child mortality rates. According to the World Bank, 12 out of 20 countries in Africa showed a drop in infant and child mortality after 2005, undoubtedly a result of the Millennium Development Goals.

The world’s youngest countries also tend to be the poorest and least educated. The United Nations’ Human Development Index shows that sub-Saharan African countries continue to suffer from low life expectancy rates, incomes and levels of schooling.

Timor-Leste, Uganda and Chad, the top-three youngest countries, have an average life expectancy rate of 56.26; average years of schooling, 2.77; and average annual income, $1,276. Chad, a landlocked nation, has a life expectancy rate of 51.18 years and average annual income of $1,621.77. In 2013, Chad was estimated to have the highest lifetime risk, or 1 in 15, of an adult woman dying from maternal causes.

Life expectancy in Timor-Leste, a maritime country that shares landmass with Indonesia, has an average life expectancy of 67.54 years and an average annual income of $9,673.61.

Demographers note a close correlation between socioeconomic development and a population’s vintage.

“What you find in countries is that many countries run in parallel with development and social economic growth and you start to see fertility decline,” said Kathleen Beegle, the lead economist for the Africa region at the World Bank. “Indonesia used to have high fertility, but then it fell, and you look at Vietnam and you see the same patterns. This happens with development, as women become more educated and as countries become richer.”

In addition, historical roadblocks for many of these countries include protracted civil unrest and conflicts. Timor-Leste, for example, is recovering from a war with Indonesia. Chad’s stability has been continually undermined since its independence from France in 1960, with rebel movements and autocratic leadership the norm. Chad is also a large contributor of troops to UN peacekeeping operations in West Africa, enabling Chad’s government to offer steady employment to a portion of its young male population.

A slant of sun appears over the youngest countries in their general declines in fertility rates. From 1960 to 2010, fertility dropped from 5.7 children to a smidgen above 5 per woman in sub-Saharan Africa. Certain countries, like Ethiopia, have managed a more rapid drop to 4.6 children as of 2012, from more than 7.

Nonetheless, a shift in population trends takes time and even longer to be noticeable. Demographers forecast that the top-10 youngest countries are unlikely to lose their place too soon because of increased life expectancy in developing countries.

Karen A. Woodrow-Lafield, a research professor at the University of Maryland’s Population Research Center, noted that for African overall, more than one-half of the world population is expected to be under 25 years old until 2050. Based on the latest UN population projections, the world’s youngest countries in 2050 are most likely to be Niger and Mali, with more than half their populations aged 20 or under.

Mali is a classic example of how a country besieged by internal conflict can hinder women’s access to birth control and other reproductive health services. Malian culture also still encourages, to some extent, large families, which can symbolize prosperity.

For development agencies and governments, the challenge of taming and lowering fertility rates — provided the governments possess the will and budget allocations to do so — can be achieved through training and education, which experts say can take decades to see results.

Beegle of the World Bank said that the key to reducing fertility rates through education include family counseling and contraception and reproductive services. “But you can’t educate girls overnight too; it requires 7 to 8 years at a time.”

New initiatives like FamilyPlanning2020, a global partnership partly backed by the Bill and Melinda Gates Foundation, is striving to get contraceptives to women and girls who want them in the developing world. In 2013, among the 69 countries the initiative focused on, the partnership estimated that modern contraception use by 274 million women and girls averted 77 million unintended pregnancies, two million more unintended pregnancies averted than in 2012.

In a recent BBC article, two analysts argued that an extremely young population can be both a detriment and an asset. On the downside, many of the countries on the world’s youngest list suffer from a tremendous amount of youth unemployment, which can lead to crime and illegal drug use, among other social problems.

On the positive side, this population can be viewed as a potential labor source to ignite economic growth. Beegle said that the goal was to narrow the demographic dividend so that the vast population of young people can be used as productive members of the work force.

“In the long term, the anticipated fertility declines would imply large shares of working-age population whose tax contributions would be useful in building infrastructure and old-age support,” Woodrow-Lafield said. Many developing countries, however, would need to put income tax structures in place to derive benefits.

Population experts throw in a word of caution to look beyond the view that women in poor countries must shoulder most of the burden of lowering fertility rates and getting their children through school.

“The goal ought to be to develop interventions and policies that prioritize the health and the well-being of these women, which, in turn, will have the spinoff effect of reducing fertility,” said Madhavan of the Maryland Population Research Center.

“Let us not forget that in terms of global warming and other environmental concerns,” she added, “it is the consumption patterns of the rich countries, not the fertility of poor Malian women, that has had the greatest impact thus far.”

John Penney contributed research to this article.

Amy Wu

Amy Wu, a native New Yorker, was a professional journalist for 18 years, including six years in Hong Kong. She then taught journalism from 2011-2013 at Hong Kong’s Shue Yan University, where she focused on social media and mobile journalism. Since 2013, she has been a research assistant at the Merrill College of Journalism at the University of Maryland, while pursuing a Ph.D. in journalism and writing for the South China Morning Post and The Huffington Post. She has an undergraduate degree in history from New York University and a master’s degree in journalism from Columbia University. She speaks fluent Mandarin Chinese.