Scale-sensitive urban vitality diagnosis and sub-district vitality–aging mismatch: A multi-source framework for age-friendly urban renewal in Nanjing, China

Urban renewal in mature cities increasingly faces the combined challenge of population aging and the need to improve already-built environments. However, renewal practice rarely considers how vitality patterns vary by measurement scale or where vitality deficits overlap with aging-related needs. Using Nanjing, China, as a case study, this research develops a scale-linked framework that diagnoses urban vitality at sub-district, community, and grid scales and examines vitality–aging mismatch at the sub-district scale, the finest level supported by available census age-structure data. Population, economic, and functional indicators from WorldPop, downscaled GDP, nighttime lights, POIs, and road networks are integrated through a Euclidean distance-based composite index. Results reveal strong but scale-sensitive vitality clustering, with global Moran’s I values of 0.812, 0.657, and 0.660 at the sub-district, community, and grid scales, respectively. Coarser scales emphasize core–periphery concentration, whereas finer scales expose localized hotspots, coldspots, and corridor-like structures relevant to design intervention. The vitality–aging association is weak at the citywide level, indicating that the typology should be interpreted as a screening and decision-support tool rather than evidence of strong coupling. The most robust priority is low-vitality/high-aging peripheral areas, where service deficits and aging-related renewal needs overlap. The study contributes a practice-oriented framework that links sub-district priority identification with intervention targeting at the community and grid scales, supporting age-friendly urban renewal under common demographic data constraints.