This study aimed to assess the application of Cameriere’s third molar maturity index (I3M) to select an individual of 18 years or older (adult) from younger than 18 years (minor) in a sample of Polish individuals. The final sample of 982 panoramic images aged between 15 to 24 years was analyzed. The specific cut-off value of I3M < 0.08 was analyzed. The specificity (Sp) and sensitivity (Se) for males were 91.2% and 86.2% respectively. The Sp and Se for females were 93% and 82.6% respectively. The probabilities of correctly classified individuals were 87.6% and 85.3%, and Bayes post-test probability was 96.3% and 97% in males and females respectively. Further analysis indicated the alternative cut-off value of I3M < 0.07, which improved the specificity, 95.3%, and 94.7%, and worsen correct classification, 86.5%, and 84.4%, in males and females, respectively. Our results showed that the specific cut-off point of I3M < 0.08 may be a useful tool for discriminating adults from minors in Poland.