Global population aging, an outcome of demographic transition, is impacting countries worldwide. This report focuses on developing nations with high fertility rates (at least 4 births per woman), particularly in Asia and sub-Saharan Africa. It explores the challenges and opportunities these countries encounter in harnessing the ""demographic dividend"" during the period when the working-age population grows faster than the total population, as highlighted in the World Population Prospects 2022. While Asian LDCs are concluding the transition to lower mortality and fertility, African counterparts are in early stages. Case studies from two Asian and two sub-Saharan African countries analyze the gradual shift to older age structures and its impact on realizing the demographic dividend. Examining demographic, economic, social, cultural, and historical contexts, the report assesses challenges and opportunities in anticipating and preparing for population aging. Insights and best practices from countries at various transition stages form the basis for policy recommendations, aiming to guide LDC governments in fostering sustainable development, social inclusion, and adaptation to aging societies. These recommendations leverage the temporary increase in the working-age population, creating a window of opportunity for strategic planning.