Evaluation of the radiographic visibility of root pulp in mandibular third molars has been suggested as an alternative method for estimation of legal age threshold in living individuals when the root apices are mature. Here, we assessed the accuracy of this method for age thresholds of 18 and 21 years. A sample of 463 panoramic radiographs of individuals aged between 16 and 34 years was examined. The root pulp visibility of the mandibular third molars was scored; the stages ranged from 0 to 3. A receiver operating characteristic (ROC) curve and the area under the ROC curve (AUC) were used to select optimal cut-offs for 18- and 21-year-old thresholds. As prognostic predictors, the selected cut-offs were stages 1 and 2 for the 18- and 21-year-old thresholds of both sexes, respectively. For the 18-year-old threshold, the AUC, sensitivity and specificity were 0.829, 83.1% and 66.7% in females; and 0.930, 89.4% and 90.9% in males, respectively. For the 21-year-old threshold, the AUC, sensitivity and specificity were 0.874, 72.8% and 92.0% in females; and 0.906, 85.5% and 88.2% in males, respectively. The accuracy of the method for estimating the 18- and 21-year-old thresholds ranged from moderate to high. Therefore, the method must be used in conjunction with other age estimation methods, especially to predict whether a female has reached 18 years of age.