The growing elderly population living in high density cities undergoing mass urbanization raises concerns over age-friendliness of neighborhood open space, an essential component for healthy aging-in-place as elderly tend to spend most of their time at home and immediate home environment. This study discusses factors that influence age-friendly open space design pertaining to outdoor thermal environment and physical design element for highdensity high-rise housing in hot weather, using the case of public housing developments in Hong Kong. Field measurements, observations and linear regression data analyses are used to understand dynamic thermal condition, adaptive elderly user behavior and response to planned open space. It is demonstrated that four influential factors are important to correlate with elderly residents' use of open space: 1) mean radiant temperature (MRT); 2) air temperature; 3) greenery; and 4) outdoor seating. Moreover, it is found that MRT, specifically longwave MRT, is the most influential and impacts the effectiveness of greenery and outdoor seating. In addition, the study proposes guide points to reflect the effect of each factor and shed more insight into improving age-friendly neighborhood open space design against climate-change induced heat-related health issues.