Research has established the diagnostic validity of borderline personality disorder (BPD) in adolescence. The roots of BPD often lie in childhood; however, significantly less is known about the presence and correlates of BPD traits in school-age children and whether these are comparable with those observed in adolescents. Trained psychologists administered the Childhood Interview for Borderline Personality Disorder in a cohort of 14-year-old adolescents (n = 76) and a cohort of 9-year-old children (n = 70). We compared the prevalence of BPD traits in both cohorts and investigated common psychosocial correlates (comorbidity, impaired quality of life, emotional/behavioral problems, maternal distress, and observed mother-child interaction). Children and adolescents showed no significant differences regarding the type and frequency of BPD traits. In both cohorts, BPD traits were associated with comorbidity, emotional and behavioral problems, and lower quality of life. In contrast to adolescents, children's BPD traits were not significantly related to maternal distress and showed less relations to interaction patterns. Negative maternal and dyadic behavior were associated with more BPD traits in adolescents during a conflict discussion but not during fun day planning. Our study suggests that BPD traits in children are similarly frequent as in adolescents and accompanied by psychosocial impairment. However, age-related differences were revealed, mostly indicating weaker associations with the mother-child relationship. Mother-child interaction patterns in youth seem to be especially relevant during conflict discussion and provide a target for intervention. Our study provides preliminary support for potential early detection of BPD pathology among children and encourages further study of its life span perspective. (PsycInfo Database Record (c) 2023 APA, all rights reserved).