Idiopathic pulmonary fibrosis (IPF) is a well-known age-related disease. However, much less recognized has been the aging associated pathogenesis of this disorder. As we and others previously showed that dysregulation of microRNAs (miRNAs) was an important mechanism involved in pulmonary fibrosis, the role of these molecules in this pathology in the aged population has not been investigated. In this study, by using lung fibrosis model established in old mice, we found that ablation of miR-34a protected aged animals from developing experimental lung fibrosis. miR-34a was upregulated in lung epithelial cells, but not in lung fibroblasts of aged mice, and miR-34a expression was further increased in epithelial cells of the fibrotic lungs of these old animals. We found that miR-34a induced dysfunctions in alveolar epithelial cells (AECs), as evidenced by increased cellular senescence and apoptosis and mitochondrial aberrations. More importantly, these abnormalities were attenuated in AECs of the fibrotic lungs of aged miR-34a-/- mice. We found that miR-34a targeted Sirt1, a master anti-aging regulator, and two key cell cycle modulators E2F3 and cyclin E2 in lung epithelial cells and the repression of these targets was relieved in miR-34a deficient AECs. In summary, our data suggest that elevated AEC miR-34a plays a critical role in the pathogenesis of pulmonary fibrosis in the aged population. Our study also indicates miR-34a be a more precise miRNA target for treating this disease that overwhelmingly affects the people of advanced age.