Childhood trauma is a robust developmental risk factor for later personality pathology, yet exposure does not translate deterministically into borderline personality features. The present article examines emotion regulation as a theoretically central mechanism that may explain part of this heterogeneity in young adulthood. Drawing on peer-reviewed evidence published primarily between 2024 and 2026, the paper integrates developmental psychopathology, biosocial, trauma-informed, attachment, and contemporary transdiagnostic perspectives. Recent longitudinal and cross-sectional studies converge in showing that childhood maltreatment is associated with greater emotion regulation difficulties and that dysregulation is strongly related to borderline symptoms and related functional impairment. Importantly, recent mediation work in young adults indicates that emotion regulation difficulties can carry part of the statistical association between childhood maltreatment and borderline personality symptoms, while other studies show that adaptive strategies such as cognitive reappraisal may attenuate symptom severity among trauma-exposed young adults. The article develops a testable mediation model in which childhood trauma predicts borderline personality features both directly and indirectly through emotion regulation difficulties. A rigorous empirical protocol is proposed for young adults aged 18–29 years using validated self-report measures, bootstrapped indirect-effect estimation, sensitivity analyses, and transparent reporting of trauma subtypes and relevant covariates. Methodological issues—including retrospective trauma assessment, cross-sectional mediation, common-method variance, sex and gender imbalance, comorbidity, and the distinction between emotion regulation and emotion dysregulation—are considered in detail. The synthesis supports emotion regulation as a clinically meaningful, modifiable process, while cautioning against causal claims when temporal ordering has not been established. Future longitudinal and multimethod studies should test whether emotion regulation prospectively mediates the pathway from specific and cumulative childhood trauma exposure to emerging borderline features during the transition to adulthood.