Africa's youthful population is framed both as a strategic asset and as a demographic pressure requiring fertility decline. This scoping review and documentary analysis examined what conditions convert demographic scale into development, how fertility and age structure interact with human capital, and how population-policy rationales have been historically constructed. Following PRISMA-ScR, a bounded multidisciplinary search conducted on 10 September 2026 returned 30 records; six duplicate or alternative-version records were removed, 24 unique records were screened, 13 reports underwent eligibility assessment, and 11 met eligibility criteria. Citation chaining, targeted foundational retrieval, and a handsearch of the target journal added 26 eligible scholarly records, yielding 37 sources in the evidence map. Fourteen primary or authoritative policy documents were analyzed separately using a graded evidentiary protocol. The evidence rejects both population-burden determinism and its pronatalist inverse. Fertility decline can improve birth spacing and temporarily reduce child dependency, but education, institutions, capital, productive employment, and technological capability condition whether demographic change produces prosperity. Sustained very low fertility creates later ageing and labor-supply risks. Historical records also show that twentieth-century population governance included eugenic, developmental, philanthropic, diplomatic, and explicitly national-security rationales, including strategically low-key communication, without establishing a unitary secret-society command structure. The review proposes demographic stewardship centered on capability formation, reproductive agency, institutional conversion, and intergenerational sustainability.