Pathways of urban green space in shaping population aging across China via spatial spillover and non-linear threshold effects

China is undergoing an unprecedented demographic transition, with population aging posing significant urban governance challenges. As a core component of green infrastructure, urban green spaces (UGS) are hypothesized to support elderly populations through complex spatial and non-linear mechanisms. This study integrates mediation analysis, the Spatial Durbin Model, and threshold regression to examine these relationships. The findings reveal that UGS significantly promotes elderly concentration through two distinct pathways. UGS enhances public service capacity, generating an amenity-pull effect that drives the spatial sorting of the elderly across administrative boundaries. This mechanism provides a robust justification for the observed spatial spillover effects in regional aging patterns. UGS mitigates the ecological burden, a path characterized by non-linear dynamics. The relationship between UGS and aging is moderated by a multi-dimensional threshold mechanism involving both UGS density and environmental carrying capacity. The results emphasize that in an era of high population mobility and regional integration, UGS serves as a strategic resource for collaborative governance. Policymakers should transition from isolated urban planning to cross-boundary ecological coordination, leveraging these spillover and threshold dynamics to achieve balanced regional development and support the Healthy China 2030 strategy.