Astonishing change is underway in Africa, where the population is projected to nearly double to 2.5 billion over the next quarter-century – an era that will not only transform many African countries, experts say, but also radically reshape their relationship with the rest of the world.

Birthrates are tumbling in richer nations, creating anxiety about how to care for, and pay for, their aging societies. But Africa’s baby boom continues apace, fuelling the youngest, fastest growing population on earth.

In 1950, Africans made up 8 per cent of the world’s people. A century later, they will account for one-quarter of humanity, and at least one-third of all young people aged 15 to 24, according to United Nations forecasts.

The median age on the African continent is 19. In India, the world’s most populous country, it is 28. In China and the United States, it is 38, according to the World Population Prospects 2022.  The regions are based on UN classifications. Regions with less than 1 percent of the global population are not shown.

The implications of this “youthquake,” as some call it, are immense yet uncertain, and likely to vary greatly across Africa, a continent of myriad cultures and some 54 countries that covers an area larger than China, Europe, India and the United States combined. But its first signs are already here.

It reverberates in the bustle and thrum of the continent’s ballooning cities, their hectic streets jammed with new arrivals, that make Africa the most rapidly urbanising continent on earth.

It pulses in the packed stadiums of London or New York, where African musicians are storming the world of pop, and in the heaving megachurches of West Africa, where the future of Christianity is being shaped.

And it shows in the glow of Africa’s 670 million cellphones, one for every second person on the continent — the dominant internet device used to move money, launch revolutions, stoke frustrations and feed dreams.

“It feels like the opportunities are unlimited for us right now,” said Jean-Patrick Niambé, a 24-year-old hip-hop artist from Ivory Coast who uses the stage name Dofy, as he rode in a taxi to a concert in the capital, Abidjan, this year.

Africa’s political reach is growing, too. Its leaders are courted at flashy summits by foreign powers that covet their huge reserves of the minerals needed to make electric cars and solar panels.

With a growing choice of eager allies, including Russia, China, the United States, Turkey and Gulf petrostates, African leaders are spurning the image of victim and demanding a bigger say. In September, the African Union joined the Group of 20, the premier forum for international economic cooperation, taking a seat at the same table as the European Union.

Businesses are chasing Africa’s tens of millions of new consumers emerging every year, representing untapped markets for cosmetics, organic foods, even champagne. Hilton plans to open 65 new hotels on the continent within five years. Its population of millionaires, the fastest growing on earth, is expected to double to 768,000 by 2027, the bank Credit Suisse estimates.

Dinner at Sushi Mitsuki, a new restaurant in a neighbourhood with a rising skyline in the Kenyan capital of Nairobi, starts at $200 per person.

“Africa is entering a period of truly staggering change,” said Edward Paice, the director of the Africa Research Institute in London and the author of Youthquake: Why African Demography Should Matter to the World.

He added: “An expansive crowd of Muslims in holiday dress, standing shoulder to shoulder, their hands clasped and heads bowed in prayer.

The energy in Africa contrasts with the rising uneasiness in Europe and Asia.

In many countries, historically low birthrates are creating older, smaller populations. Caregivers in Italy, which is expected to have 12 per cent fewer people by 2050, are experimenting with robots to look after the aged. The prime minister of Japan, where the median age is 48, warned in January that his society was “on the verge” of dysfunction.

Africa’s challenge is to manage unbridled growth. It has always been a young continent – only two decades ago the median age was 17, but never on such a scale. Within the next decade, Africa will have the world’s largest workforce, surpassing China and India. By the 2040s, it will account for two out of every five children born on the planet, says UN World Population Prospects 2022.

Experts say this approaching tide of humanity will push Africa to the fore of the most pressing concerns of our age, like climate change, the energy transition and migration. But it has also exposed the continent’s gaping vulnerabilities.

Africa’s soaring population is partly a result of remarkable progress. Africans eat better and live longer than ever, on average. Infant mortality has been halved since 2000; calorie intake has soared. But while a handful of African countries are poised to ride the demographic wave, others risk being swamped by it.

Nigeria, Africa’s most populous nation, is already deeply stressed: Nearly two-thirds of its 213 million people live on less than $2 a day; extremist violence and banditry are rife; and life expectancy is just 53, nine years below the African average.

Yet Nigeria adds another five million people every year, and by 2050 is expected to overtake the United States as the world’s third most populous country.

Young Africans are better educated and more connected than ever: 44 per cent graduated from high school in 2020, up from 27 per cent in 2000, and about 570 million people use the internet. But finding a good job, or any job, is another matter.

Up to one million Africans enter the labour market every month, but fewer than one in four get a formal job, the World Bank says. Unemployment in South Africa, the continent’s most industrialised nation, runs at a crushing 35 per cent.

In countries like Somalia, Mozambique and Mali, opportunity-starved youths pick up guns to fight for jihad, or for money. In Gabon and Niger, youngsters fed up with sham politics crowd streets and stadiums to yell slogans in favour of military coups.

On the high seas, smugglers’ boats make perilous journeys to Europe and the Middle East, carrying desperate young Africans and their dreams of a better future. At least 28,000 have died on the Mediterranean since 2014, the United Nations says.

“Our generation takes things personally,” said Keziah Keya, a 21-year-old software engineer from Kenya. Ms Keya exemplifies the potential of that generation. Born into a poor family, she taught herself to code using the internet, and later represented Kenya at the International Math Olympiad in London. Last year, she was hired by a renewable energy company.

But she recently watched in dismay as a river near her home ran dry. Soon after, her grandmother’s crop of tomatoes withered. Starving cattle began to die. Three local herders took their own lives, she said.

“If we want to change things, we have to do it ourselves,” said Ms Keya, who last month flew to Bryn Mawr College in Pennsylvania to study computer science, on a full scholarship. But she sees her future in Kenya. “We can’t afford to wait.”

Forecasting population trends is a fraught and contentious business, with a history of flawed predictions. In the 1970s, books like The Population Bomb by Paul R. Ehrlich popularised fears that an overcrowded planet would lead to mass starvation and societal collapse.

Africans are rightfully cautious of foreigners lecturing on the subject of family size. In the West, racists and right-wing nationalists stoke fears of African population growth to justify hatred, or even violence. But experts say these demographic predictions are reliable, and that an epochal shift is underway. The forecasts for 2050 are sound because most of the women who will have children in the next few decades have already been born. Barring an unforeseeable upset, the momentum is unstoppable.

“It’s the mother of all megatrends,” said Carlos Lopes, an economist from Guinea-Bissau who formerly headed the UN Economic Commission for Africa.

Many others agree. The economic rise of China and India were the first great shocks of this century, they say. Africa’s youthful tide will most likely drive the next seismic shift. Its first tremors are already being felt, and nowhere more than in global culture.

• The New York Times report