This study examined implicit theories of immaturity among 345 Canadian sixth- and ninth-graders. Qualitative analysis of adolescents' descriptions of an immature peer revealed six foci to implicit theories of immaturity. The majority of adolescents' descriptions focused on childlike behaviors, silly/goofy behaviors, or on mean/hurtful behaviors. Sixth-graders' theories were more likely to describe childlike behavior, whereas ninth-graders' theories were more likely to describe silly/goofy behavior. (Author/KB)