Patient's age at the time of diagnosis is an important prognostic factor for differentiated thyroid cancer (DTC) as reflected in various staging and risk stratification systems. However, discrepancies exist among the different staging systems on an optimal cut-off age for predicting the clinical outcome of patients with DTC. To determine the age at diagnosis most predictive of clinical outcomes of DTC, a population-based cohort study was performed composed of 35,323 patients with DTC between 1988 and 2010 using the Surveillance, Epidemiology, and End Results (SEER) database. The Youden index J was used to determine the most predictive age-at-diagnosis for thyroid-cancer-specific death. The multivariate Cox proportional hazards model was used to determine the hazard ratios (HRs) for each age group. With a median follow-up of 5.4 years (range, 0-22.9 years), DTC-associated mortality was 1.5% (n = 533) and the rate of death from overall cause was 7.0% (n = 2482). The optimal cutoff age at diagnosis for thyroid-cancer-specific death was 57. Multivariate analysis found that the age-at-diagnosis is the most prognostic factor for thyroid-cancer-specific death (HR 10.02, 95% CI 8.18-12.28). Age at diagnosis is the most important prognostic factor for DTC patients. Based on our analysis, age at diagnosis of 57 might be the optimal predictor of thyroid-cancer-specific death. This finding might be used as consideration in revision of the risk stratification system for treatment of DTC patients.