Spatiotemporal distribution pattern and mechanism analysis of city-level happiness based on facial cognition in China: An age difference perspective

City-level happiness is a key indicator for evaluating the quality of urban development. However, most existing studies rely on subjective surveys, which often fail to reflect the perceptual differences among diverse age groups. This study constructs a city-level happiness index using facial expression recognition from social media images, supplemented by survey data. From the perspective of age differences, we systematically examine the spatiotemporal patterns and influencing mechanisms of city-level happiness across China from 2021 to 2023. The results show that average city-level happiness follows the order of young adults > middle-aged adults > minors > older adults. The cold spots for city-level happiness are concentrated in western and northwestern China, accompanied by age-related differences among hotspots. Compared with minors, hotspots for young and middle-aged adults are more southerly. Housing prices, income, and consumption are the most stable and significant factors, which positively contribute to city-level happiness across all groups. Education resources are particularly important for minors. Infrastructure and transportation significantly influence young and middle-aged groups. Environmental factors have a greater positive impact on older populations. This study reveals the heterogeneous characteristics and multifaceted determinants of city-level happiness from an age-specific perspective. The findings provide new empirical evidence for inclusive urban planning and contribute to policy formulation aimed at enhancing happiness for all age groups.