Background: The incidence of rotator cuff tears in young patients has increased recently with the increase in sports and recreation activities, however, few studies have reported clinical outcomes after arthroscopic repair of large to massive rotator cuff tears in young patients. This study aimed to evaluate preoperative characteristics and postoperative outcomes after arthroscopic repair of large to massive rotator cuff tears in patients younger than 60 years, and to compare these results with those of tear size-matched patients older than 60 years. Methods: Forty-eight patients who underwent arthroscopic repair for large to massive rotator cuff tears were included. Group I (n = 24) consisted of patients younger than 60 years, while tear size-matched patients older than 60 years were assigned to group II (n = 24). Clinical outcomes were evaluated preoperatively and at 3, 6, and 12 months postoperatively, and at the final visit. All patients underwent magnetic resonance imaging (MRI) preoperatively and at 3 and 12 months postoperatively to evaluate repair integrity. Results: The mean patient age was 53.4 ± 4.2 years in group I and 67.4 ± 4.5 in group II (P = 0.001). At the last visit, there were no significant differences in postoperative clinical scores or passive range of motion between two groups. However, young patients complained of shoulder pain less frequently than elderly patients (visual analog scale for pain at last visit: 0.8 ± 0.3 in group I, 2.4 ± 1.9 in group II, P = 0.04). Elderly patients showed more advanced fatty infiltration preoperatively than young patients and advanced fatty infiltration was correlated with postoperative repair integrity (r = 0.83, P = 0.001). Postoperative re-tear rate was 12.5% in group I, and 33.3% in group II (P = 0.08). Conclusion: More satisfactory clinical and structural outcomes followed by less advanced preoperative fatty infiltration can be expected in younger patients compared with patients older than 60 years, based on large to massive rotator cuff tear treatment outcomes.