The evaluation of the ossification of the medial clavicular epiphysis being part of an assigned expert approach according to standard plays an important role within civil and criminal proceedings in assessing whether a person has reached her/his 19th or 22nd year of age. Evaluation of the medial clavicular epiphysis with thin-section CT is one of the methods recommended by the Study Group on Forensic Age Diagnostics of the German Association of Forensic Medicine. In this retrospective study, we evaluated the thin-section CT (section thickness of 0.6 and 1 mm) images of 254 patients (146 male, 108 female) with an age range of 13-28 years according to the Kellinghaus substage system. The mean values of female patients were observed to be about 10 months lower for stage 2a than the mean values of the male patients, about 13 months lower for stage 2b, and about 18 months lower for stage 2c. The earliest appearance for stage 3c was at 19 years in both sexes. Our data from this study were consistent with both our previous studies and the data of other studies. We think that stage 3c is important in determining whether a person has reached the age of 18 or not and, therefore, that the Kellinghaus substage system is a requirement in the assessment of forensic age.