Objectives To study the relationship between the union of the epiphyseal plate of the medial clavicle and age in the Thai male population. Methods Age-at-death estimation was evaluated from clavicles obtained from 200 cadavers of Thai males aged between 11 and 35 years. After soft tissue removal, the fusion status of the medial clavicular epiphysis was classified using a morphological classification. Stages of the epiphyseal union were categorized as follows: 1) nonunion with no epiphyses, 2) nonunion with separate epiphyses, 3) partial union, and 4) complete union. Results The stage of development of the epiphyseal union increased with age. The nonunion stage was found in indivi-duals up to 22 years old. Complete epiphyseal union was found as early as age 22. Partial union was found in individuals 19-26 years old. Conclusions The stage of the epiphyseal union of the medial clavicle in Thai males is related to age. We recommend using complete union (stage 4) of epiphyseal development to indicate age more than 22 years and nonunion (stage 1 and 2) to represent age less than 22 years in Thai males.