If the proportion of younger age groups is steadily declining while the proportion of older age groups is increasing, what change is occurring in the age structure?

Correct answer: Population is moving toward ageing

When the share of children falls and the share of older people rises, the age distribution is shifting toward older ages. This is called population ageing. It commonly develops after fertility remains low for a long period, because fewer children enter the population, while people who survive to older ages form a larger proportion. Better health and longer life expectancy can strengthen this pattern.

Option B directly describes this change. Option A says the opposite, because a youthful population would have a relatively large share of children. Option C is not the best description: child dependency may fall, but the central change is the ageing of the whole population. Option D cannot be concluded from age shares alone and would usually conflict with a sustained fall in younger cohorts. Therefore B is correct.