Age is an important prognostic factor of papillary thyroid carcinoma (PTC). In this study, we investigated the prognosis and prognostic factors of PTC in patients younger than 20 years. We enrolled 110 patients who underwent initial surgery at Kuma Hospital between 1987 and 2008. Tumor size > 4 cm, metastatic node ≥ 3 cm, and significant extrathyroid extension were more frequently detected in 8 patients with distant metastasis at diagnosis than in 102 patients without distant metastasis. Ten- and 20-year lymph node recurrence-free survival (LN-RFS) and distant recurrence-free survival (DRFS) rates were 84 and 80%, and 95 and 89%, respectively. Metastatic node ≥ 3 cm, age ≤ 16 years, tumor size > 4 cm, and male gender affected LN-RFS, and the former two had an independent prognostic value in multivariate analysis. Metastastic node ≥ 3 cm, significant extrathyroid extension, age ≤ 16 years, tumor size > 4 cm, and a male gender predicted a poor DRFS, and the former two were independent prognostic factors. To date, only 2 patients have died of PTC. These findings suggest that, in the subset of PTC patients younger than 20 years, metastatic node ≥ 3 cm, significant extension, and age ≤ 16 were important signs of aggressiveness of carcinoma, and careful treatment is necessary for patients with these characteristics, although the cause-specific survival was excellent.