Nigeria

Its median age of 18.1 years is the lowest on this list, driving a youthful expansion that adds roughly 5.4 million people yearly. This explosive growth outpaces the average global population growth rate of 0.9% by nearly threefold, yet the fertility rate has declined from 6.4 in 1980 to 5.1 births per woman. If current trends hold, Nigeria's population will double to over 400 million by 2050, reshaping global rankings and straining urban infrastructure in megacities like Lagos.