Criterion D of the Diagnostic and Statistical Manual of Mental Disorders (5th ed.; DSM–5; American Psychiatric Association [APA], 2013) refers to a possible onset of personality disorders (PDs) in adolescence and in Section II the development/course in adolescence is described by some typical characteristics for several PDs. Yet, age-specific expressions of PDs are lacking in Section III. We urgently need a developmentally sensitive assessment instrument that differentiates developmental and contextual changes on the one hand from expressions of personality pathology on the other hand. Therefore we investigated which items of the Severity Indices for Personality Problems-118 (SIPP-118) were developmentally sensitive throughout adolescence and adulthood and which could be considered more age-specific markers requiring other content or thresholds over age groups. Applying item response theory (IRT) we detected differential item functioning (DIF) in 36% of the items in matched samples of 639 adolescents versus 639 adults. The DIF across age groups mainly reflected a different degree of symptom expressions for the same underlying level of functioning. The threshold for exhibiting symptoms given a certain degree of personality dysfunction was lower in adolescence for areas of personality functioning related to the Self and Interpersonal domains. Some items also measured a latent construct of personality functioning differently across adolescents and adults. This suggests that several facets of the SIPP-118 do not solely measure aspects of personality pathology in adolescents, but likely include more developmental issues.