Forensic age estimation, after completion of third molar mineralization, regressive features such as apposition of secondary dentin, which is seen as narrowing of the pulp space in the radiographs, can be used as an alternative. In the present study, we explored the radiographic visibility of the root pulp of mandibular first molars using stage classification of Olze et al. (Int J Legal Med 124(3):183-186, 2010) in a sample of 901 orthopantomograms (404 males and 497 females) of south Indian origin with age ranging from 14 to 22 years. Descriptive statistics for the stages according to age was performed for both sexes separately. The strength and direction of the relationship between the chronological age and pulp visualization stages was tested using spearman's rho correlation statistics. The relationship between age and stage attainment showed statistical significance for both sexes. A strong, positive correlation was seen between the stage and chronological age. All males and females presenting stage 0 of root pulp visibility were younger than 18 years. If stage 1 is determined, it is highly possible that an individual regardless of sex is younger than 18 years. Stage 2 was attained in 79.6% males and 83.1% females who were at least 18 years. One hundred percent males and 92.8% females with stage 3 were at least 18 years. The accuracy of this method in mandibular first molars for estimating age threshold of 18 years ranged from moderate to high. However, it is recommended to use this method in conjunction with other age estimation methods.