The question of whether an individual has reached the age of 18 is of crucial importance in forensic age estimation practice. In some countries, the age threshold of 21 years is relevant as well. A completed mineralization of third molars is not a sufficient criterion for a diagnosis of a minimum age of 18 years with the required probability. In a material of 1,198 orthopantomograms from 629 females and 569 males aged between 15 and 40 years, the visibility of the root pulp of fully mineralized lower third molars was evaluated according to stages 0, 1, 2, and 3. In females, stage 0 was first noticed at age 17.2 years, in males at age 17.6 years. In either sex, the earliest observation of stage 1 was between 21.0 and 22.4 years. Stage 2 was first achieved by males between 22.3 and 22.7 years, by females between 23.4 and 24.7 years. The occurrence of stage 3 was first found in both sexes between 25.1 and 25.9 years. These findings indicate that for stage 0, an age below 18 years cannot be excluded. However, for stage 1, the examined individual must be over 18 years of age and most probably over 21 years of age. For stages 2 and 3, the age can safely be stated to be over 21 years of age. This method may be a powerful tool for forensic dentists in age estimation in asylum and criminal proceedings.