The current diagnostic system suggests that personality disorder categories be applied to children and adolescents in rare circumstances because of expected changes in personality pathology across development. The present study examined the stability in personality pathology, specifically psychopathy, across childhood and adolescence. Using a short form of the CPS and mixed models incorporating fixed and random effects, we examined the reliability, individual stability, mean-level stability, and predictive utility of juvenile psychopathy as a function of age (i.e., from 7 to 17 years old) in over 1,500 boys from the three cohorts of the Pittsburgh Youth Study. If adolescent development contributes to instability in personality pathology, large age-related fluctuations in reliability, stability, and predictive utility should be observed, particularly in the latter part of adolescence when normative changes are hypothesized to influence levels of psychopathy. Such fluctuations were not observed. In general, juvenile psychopathy could be reliably assessed beginning in childhood, was fairly stable across short and long intervals, showed little mean-level fluctuation, and predicted delinquency across adolescence. These results suggest that concerns about large changes in personality pathology across childhood and adolescence may be overstated. Implications and future directions are discussed.