If a country's birth rate remains low for a long time, which age group's share is likely to become smaller?

Correct answer: Child and young age groups

When the birth rate remains low for many years, each new birth cohort contains fewer children. As these small cohorts grow up, the proportion of young people also becomes smaller. This effect does not mean that the number of children must fall every single year, because migration and the total population can change, but their share is likely to decline over time. Age composition changes gradually as cohorts move upward.

Option A is correct because children and, later, young adults come from recent birth cohorts. A low birth rate directly reduces the supply of children and eventually reduces the young-age share. The elderly share need not become smaller; it may rise if people live longer. The middle-age share may change in different ways, and all groups will not become equal. Thus A is the only option that follows consistently from sustained low fertility.