Impaired healing of adult tendons with fibrosis remains clinical challenges while neonatal tendons have full functional restoration. However, age-associated cellular and molecular changes in tendon cells and tendon stem/progenitor cells (TSPCs) remain unknown. Here, comparative single cell transcriptomics of early postnatal (2 weeks old) and adult (20 weeks old) mouse tendons revealed that adult tendons have reduced number of TSPCs, decreased gene expression in tendon and cartilage development, and a greater population of fibro-tenogenic cells. Notably, adult TSPCs and tenocytes exhibit increased expression of immune-response and oxidative-stress genes with higher EGFR but decreased IGF signaling. Adult tendon cells show increased levels of intracellular reactive oxygen species (ROS) in vivo. In contrast, antioxidant treatment of adult tendons significantly reduces intracellular ROS of TSPCs and improves tendon strength in vivo. Hence, these findings suggest that increased inflammation and ROS during tendon aging deteriorates tendon function and regeneration that can be mitigated by antioxidant treatment.