Estimation of age in individuals, either living or dead, is a daunting task in forensic analyses. The relative visibility of the periodontal ligament of the mandibular third molars, after the root formation which is completed in four stages, has been identified as having the potential to classify the subject as being above or below 18 years. A total of 339 orthopantomograms (OPGs) (180 males, 159 females) from the Indian population belonging to the age group of 15–40 years, were evaluated for the stage of periodontal ligament visibility and scored. The chronological age and the periodontal ligament visibility staging (0, 1, 2, 3) demonstrated a strong positive correlation. The grading of Stages 1, 2 and 3 was evidenced at age >18 in both males and females with both right as well as left third molars with a classification accuracy of 100%. Among Indians, it can be preliminarily concluded that periodontal ligament visibility from Stage 1 onwards can be used to define the 18-year threshold, i.e. state that males or females are above 18 years of age.