The aim of this study was to determine differences in maturation stages of the mandibular third molars (MTM) of Chilean individuals by age, sex, and side. The maturation stage of 930 lower third molar teeth were analysed, in patients aged between 9 and 24 years (432 males and 498 females), by panoramic radiography. The stage of maturity of the MTM was classified from A to H, following the method established by Demirjian. Multinomial logistic regression was performed for maturation stage by age. The lowest age of complete maturity of the MTM was 16 years in both sexes (Stage H). The correlation between maturation stage and sex was significant, positive, and high for both sexes, in both tooth 48 and tooth 38, showing that increasing age and maturation stage are correlated. In both males and females, maturation Stage H of the MTM was associated with the age of 18 years. The fit of the model was significant, showing that it has a greater predictive power than the null model. Nagelkerke’s pseudo R2 explained 73.5% of the variations of the model. In the contemporary Chilean population, if the maturity of the MTM is at a stage prior to complete calcification of the crown (Stage D), it is very unlikely that the individual is aged 18 years or over. It is highly probable that a Chilean individual with complete root formation of the MTM is legally of age, regardless of side or sex.