In dental age diagnostics radiological evaluation of third molars is the most important criterion to establish whether an individual has attained 18 years of age. However, completed root development of third molars as the only criterion is insufficient for an establishment of legal certainty. In the present paper it was investigated whether root development is slowed down in impacted lower third molars and thereby increases the probability that 18 years of age has been attained. By means of logistic regression a delay of 0.6 years in male subjects and 0.7 years in female subjects was shown in impacted third molars with completed root development compared to cases with erupted third molars. There was no case in which an individual with impacted lower third molars with completed root development on both sides was under the age of 18. In the presence of impacted third molars with completed root development on one side only one subject was under 18 years of age (17 years and 11 months!). Conclusion: If two impacted third molars with completed root development in the lower jaw are determined in dental age diagnostics, a minimum age of 18 years in central Europeans can be suspected beyond any reasonable doubt. If one impacted mandibular third molar with completed root development is present it is very likely that 18 years of age have been attained.