This question assesses the understanding of demographic changes. The correct answer is A because a decrease in marriage and fertility rates typically leads to an aging population. Option B is incorrect because while there might be some impact on the demand for schools, it is not the direct consequence of the rates decreasing. Option C is incorrect because there is no direct correlation between marriage and fertility rates and crime rates. Option D is incorrect because a decrease in fertility does not immediately lead to a decrease in the workforce, which includes individuals across a wide age range.