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Differentiated thyroid carcinomas (DTC) are the only tumors for which age is a determinant of stage in the American Joint Committee on Cancer's (AJCC) staging protocol. In this study, we re-examined the relationship between age, extent of disease, and prognosis by using a large dataset with longer follow-up times.We examined the Surveillance, Epidemiology, and End Results (SEER) registry data 1973 to 2005 for patients with DTC as their only known malignancy. We used Cox multivariate analyses to generate mortality hazard ratios, controlling for several variables, to evaluate the effects of age and disease extent.We identified 55,402 patients with DTC. Of these, 49,240 had sufficient data to generate a TNM stage on the basis of AJCC guidelines. Within stage II, younger patients (<45 years) have worse outcomes than older patients (P < .001). Younger patients had an 11-fold increase in mortality between stages I and II, whereas there was no difference for older patients. When we uniformly applied the 45-and-older staging protocol to all patients, we found that stages III-IVc had a significantly greater risk of mortality for all patients compared with stage I.The presence of regional and metastatic thyroid cancer bears prognostic significance for all ages. Under current AJCC guidelines, young patients with metastatic thyroid cancer may be understaged.