Stability of alexithymia is low from adolescence to young adulthood, and the consistency of alexithymia is associated with symptoms of depression and dissociation

•The Youth Well-being Study followed 755 adolescent students for six years.•The stability of alexithymia was lower than previously recorded among adolescents.•Levels of alexithymia decreased from adolescence to young adulthood.•Mental health symptoms were associated with the consistency of alexithymia.•Poor relationships with peers and loneliness associated with alexithymia levels.