Declining birth rates shift population towards older ages

Steps:

• Define ageing population as one with a rising proportion of elderly relative to youth.
• Identify factors affecting age structure: birth rates add young people, death rates remove all ages, retirement/pensions influence workforce but not demographics directly.
• Analyze each option's impact on age distribution assuming other factors constant.
• Conclude that only reduced births lower the youth share, ageing the population.

Why A is correct:

• A fall in the birth rate reduces new young entrants, increasing the elderly's share of the total population per demographic transition theory.

Why the others are wrong:

• B: A rise in the death rate reduces overall population but disproportionately affects the elderly, potentially lowering their proportion.
• C: An increase in the age of retirement extends working years but does not alter birth or death rates affecting age structure.
• D: An increase in the state pension affects economic incentives for retirement but leaves demographic age distribution unchanged.

Final answer: A