Characteristics and driving factors of population age structure in China: A study based on the scale nesting theory

China’s population age structure (PAS) has already entered a modern phase, with the effects of aging and declining birth rates becoming increasingly severe. These trends pose significant challenges to demographic and socioeconomic stability. In this study, the scale nesting theory was used to analyze the population census data for 2000–2020 from prefecture-level cities. Using a multiscale geographically weighted regression model, this study explores the spatial characteristics of China’s age structure and quantitatively examines the driving factors within this nested framework. The evolution of PAS in the prefecture-level cities in China exhibited distinct stages, while depicting a rapid transition toward an aging model. In the nested space, single-scale nesting was primarily synchronous, while double-scale nesting was characterized by simultaneous synchronization. The Northeast region exhibited the highest degree of synchronous development in the PAS within the nested space. The regression coefficients indicated that the population system and per capita gross domestic product (GDP) were the primary and secondary factors, respectively. Topographical variation exerted an influence on only the synchronous-advance type, while PM_2.5 exhibited a significant association with the advance-lag type. This study provides scientific support for high-quality development across regions in the context of heterogeneous population age structures.