Many studies have shown associations between air pollution and asthma admissions in Korea, but have not reported whether these effects differ by age classification. The purpose of this study was to determine whether air pollution effects on asthmatic hospital admissions are different by three age groups (years): children (less than 15), adults (15–64; reference group), and the elderly (over 65). Daily time-series data from seven metropolitan cities in South Korea were analyzed in two stages. In the first stage, relative asthma morbidity rates associated with air pollution were estimated for each city and age group, using semiparametric log-linear regression. In the second stage, estimates from all seven cities were combined by age group using Bayesian hierarchical modeling. The effects of exposure to particulate matter <10 μm in aerodynamic diameter (PM10), carbon monoxide (CO), and nitrogen dioxide (NO2) varied significantly by age groups. Using adults as the referent, the relative rate (RR) of asthma admissions with 10 μg/m3 increase of PM10 is 1.5 % (95 % CI 0.1–2.8 %) lower for children and 1.3 % (95 % CI 0.7–1.9 %) higher for the elderly; RR with 1 ppm increase of CO is 1.9 % (95 % CI 0.3–3.8 %) lower for children; RR with 1 ppb increase of NO2 (1 ppb) is 0.5 % (95 % CI 0.3–0.7 %) higher for the elderly. No significant age group difference in relative rate was found for ozone or sulfur dioxide.