A country experiences high fertility for a long period and then fertility falls sharply. Which age-structural change is most likely over the next two decades?

Correct answer: Large earlier young cohorts will enter working ages while the child share declines

The governing concept is cohort movement through the age structure. A long period of high fertility creates unusually large groups of children and young people. When fertility then falls sharply, the newly born cohorts become smaller, so the child share and the base of the age pyramid begin to narrow. Meanwhile, the large cohorts born earlier advance into working ages during the following years and decades. Thus, option A correctly describes the likely transition. Option D is opposite to the expected effect of falling fertility. Options B and C are also impossible because demographic groups do not disappear instantly; they age gradually. This transition may create a demographic dividend, but only if education, health, skills and employment opportunities can absorb the expanding working-age population.