The association between long-term exposure to ambient fine particulate matter with aerodynamic diameter ( ) and lung function in young people remains uncertain, particularly in Asia, where air pollution is generally a serious problem. This study investigated the association between long-term exposure to ambient and lung function in Taiwanese children, adolescents, and young adults. This study comprised 24,544 participants 6-24 years of age, with 33,506 medical observations made between 2000 and 2014. We used a spatiotemporal model to estimate concentrations at participants' addresses. Spirometry parameters, i.e., forced vital capacity (FVC), forced expiratory volume in 1 s ( ), and maximum midexpiratory flow (MMEF), were determined. A generalized linear mixed model was used to examine the associations between long-term exposure to ambient and lung function. The odds ratios (ORs) of poor lung function were also calculated after adjusting for a range of covariates. Every increase in the 2-y average concentration was associated with decreases of 2.22% 95% confidence interval (CI): , , 2.94 (95% CI: , ), and 2.79% (95% CI: , ) in the FVC, , and MMEF, respectively. Furthermore, it was associated with a 20% increase in the prevalence of poor lung function (OR: 1.20; 95% CI: 1.12, 1.29). Two-year ambient concentrations were inversely associated with lung function and positively associated with the prevalence of poor lung function in children, adolescents, and young adults in Taiwan. https://doi.org/10.1289/EHP5220.