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Compounded by rapid urbanization and climate change, urban heat stress increasingly threatens public health and compromises the livability of cities globally. While urban morphology fundamentally shapes outdoor thermal comfort, the physical mechanisms driving age-stratified heat exposure and distributive inequality remain poorly quantified, limiting equity-oriented climate adaptation. Here, we develop an age-stratified heat exposure equity (ASHEE) analytical framework to evaluate summer mean radiant temperature (Tmrt) and intragroup heat inequality across 81 Chinese cities. We uncover a pronounced generational exposure-inequality paradox: adults endure the highest heat exposure (55.20 ± 4.04 °C) compared to children and the elderly, whereas the elderly experience the most severe distributive disparity (Gini index: 0.195 ± 0.055). Across all cities, sky view factor (SVF), vegetation coverage (VegC), and biomass density (BioD) account for over 50% of this inequality. We identify the non-linear thermal response thresholds for key urban morphological factors, providing quantitative insights for urban planning. Generally, relative to the model baseline, SVF < 0.56, VegC > 0.35, and BioD > 1.36 m3/m2 are associated with more favorable radiant-thermal conditions, with strongest negative contributions to Tmrt were –8.33 °C, –3.72 °C, and –5.17 °C, respectively. These thermal regulatory patterns differ modestly across geographic contexts. Our findings advance urban climate adaptation by enabling threshold-guided, human-centric spatial optimization, offering a scalable pathway to foster urban heat equity and sustainability.