Since the Industrial Revolution in the 18th century, global modernization has rendered population aging an unavoidable demographic trend in both advanced and developing economies. Although this trend signals societal advancement, aging populations pose considerable challenges to economic vitality, welfare system sustainability, and intergenerational equity. Understanding how demographic aging structurally unfolds within modernization processes—and critically evaluating how different polities have responded—holds direct relevance for China’s ongoing policy formulation. This study adopts a convergent mixed-methods approach. Quantitatively, it analyzes cross-national panel data (1960–2020) from World Bank and United Nations sources, applying growth curve models to examine associations between stages of modernization and aging trajectories across G20 countries. Qualitatively, guided by a most-similar/most-different case selection logic, it examines national policy documents from selected modernized states. A thematic analysis is used to extract institutional designs concerning pension reform, long-term care, and older workforce participation. The analysis draws on intergenerational justice theory and welfare state typology to interpret the drivers and consequences of distinct policy choices. By situating China’s experience within global modernization patterns, this study identifies both universal aging challenges and China’s particular institutional and cultural context. It concludes by proposing evidence-based, context-sensitive policy recommendations for fostering inclusive and sustainable responses to population aging in China.