Objective: The current study compares diagnostic performances of Demirjian’s mineralization stage (DS) and Cameriere’s third molar maturity index (I3M) on estimating adulthood in Turkish individuals and to investigate a more suitable cutoff value in a sample Turkish population. Materials and Methods: A retrospective study was conducted on a sample of digital panoramic images of 512 healthy individuals aged between 14 and 23 years old. The diagnostic capabilities were tested with receiver operating characteristic (ROC) curves, sensivities, specificity were evaluated and post-test probabilities were calculated with Bayes’s theorem. Results: The areas under the ROC were 0.88 for DS and 0.89 for I3M. The sensitivity and the specifities of H stage were 41.4%, 97.9% for females and 64.3%, 93.4% for males, and I3M<0.08 were 46.8%, 97.4% for females and 71.4%, 92.6% for males. The cut-off of the dataset I3M<0.2 performed better for females, increasing sensitivity to 73.9%, post-test probability to 80.9% while decreasing specificity to 87.2%. Conclusion: I3M<0.2 performed better for females with higher sensitivity values while lowering the highest specificity values. Concerning legal rights and ethics, I3M<0.08 or H stage provide the most accurate and consistent results in adulthood determination in males, whereas in females I3M<0.08 and I3M<0.2 provide sufficient results when a test with the highest specificity is sought.