Purpose In the Study of (E7080) Lenvatinib in Differentiated Cancer of the Thyroid (SELECT), lenvatinib significantly prolonged progression-free survival (PFS) versus placebo in patients with radioiodine-refractory differentiated thyroid cancer (RR-DTC). This prespecified subanalysis investigated the effect of age on the efficacy and safety of lenvatinib. Patients and Methods This randomized, double-blind, phase III study enrolled patients with histologically confirmed RR-DTC stratified by age (≤ 65 or > 65 years). Patients (N = 392) received lenvatinib 24 mg/day (n = 261) or placebo (n = 131). The primary end point was PFS; secondary end points included overall survival (OS), objective response rate, and safety. Results In both treatment arms, median ages were 56 (younger group) and 71 years (older group). PFS benefit was maintained with lenvatinib versus placebo in the younger and older age groups, with median PFS of 20.2 versus 3.2 months (hazard ratio [HR], 0.19; 95% CI, 0.13 to 0.27; P < .001) and 16.7 versus 3.7 months (HR, 0.27; 95% CI, 0.17 to 0.43; P < .001), respectively. PFS did not differ with age in either treatment arm. OS was improved in older lenvatinib-treated patients versus placebo (HR, 0.53; 95% CI, 0.31 to 0.91; P = .020). Younger lenvatinib-treated patients showed significantly higher ORR (72% v 55%; P = .0038), longer time to first dose reduction (3.7 v 1.5 months), and lower proportion of grade ≥ 3 treatment-related adverse events (67% v 89%; P < .001) compared with older patients. Conclusion This subanalysis demonstrated improved PFS with lenvatinib treatment versus placebo in both age groups, although higher toxicity was observed in older patients. Despite the allowance of crossover after disease progression, the OS benefit was observed in older patients, suggesting that lenvatinib should be considered for treatment of patients of any age with RR-DTC.