Subjective well-being (SWB) of older adults could be affected by both individual and community characteristics. However, the effect of community characteristics, such as population aging in local areas, remains unclear. This study examined the cross-sectional and longitudinal associations between the area-level population aging and SWB of older individuals from two distinct surveys. Those analyzed were 572 respondents aged 75 years and older for a cross-sectional survey in a metropolitan area in Tokyo, Japan (Study 1) and 1,257 and 859 respondents for a cross-sectional and longitudinal analysis, respectively, for a 2-year longitudinal survey project in urban and rural areas of Fukui Prefecture (Study 2). Area-level population aging was assessed by the number of people aged 65 years or older per 100 residents. SWB was assessed with the Life Satisfaction Index-A (LSIA). Multilevel analysis was performed to examine unconditional and conditional associations between the area-level number of older adults per 100 residents and the individual-level LSIA scores. The area-level number of older adults per 100 residents was significantly and positively associated with the LSIA scores in Study 1 (p = 0.042), even after controlling for the area- and individual-level covariates. In Study 2, we also found a significant effect of the area-level number of older adults per 100 residents on LSIA scores in the longitudinal multivariate analysis (p = 0.049). Findings from two survey projects suggested cross-validity in the positive effect of area-level population aging on older adults’ SWB. Policymakers should consider older citizens' SWB in the recent urban-to-rural migration governmental policy as well as in urban renovation planning.