The 20-item Toronto Alexithymia Scale (TAS-20; Bagby, Parker, & Taylor, 1994; Bagby, Taylor, & Parker, 1994) is the most widely used self-report measure of the alexithymia construct. The TAS-20 comprises 3 factors that assess difficulty identifying feelings, difficulty describing feelings, and externally oriented thinking. Although the instrument is being increasingly used with adolescent respondents, the psychometric properties of the TAS-20 have not been systematically evaluated in preadult populations. In the present study, we examined measurement invariance of the factor structure, internal reliability, and mean levels of responses on the TAS-20 in groups of younger adolescents (aged 13-14 years), middle adolescents (aged 15-16 years), and older adolescents (aged 17-18 years), as well as in a comparison group of young adults (aged 19-21 years). Formal readability analysis of the TAS-20 assessment was also conducted. Results revealed systematic age differences in the factor structure and psychometric properties of the TAS-20, with the quality of measurement progressively deteriorating with younger age. Much of this effect could be attributed to the reading difficulty of the scale. The use of the TAS-20 with teenage respondents is not recommended without appropriate adaptation and further psychometric validation. Several adaptation strategies are discussed.