The aim of this study was to identify associations between borderline personality (BP) traits and reactive and proactive aggression, and to compare the meditational effects of maladaptive coping in samples of older adolescents (n = 133) and young adults (n = 93), which has not hitherto been explored. This was a cross‐sectional study that used self‐report measures to assess BP traits on a continuum, trait‐based reactive and proactive aggression, and coping strategies. In adults, maladaptive emotional coping significantly mediated the relationship between BP and reactive aggression, and maladaptive avoidant coping mediated the relationship between BP and proactive aggression; no significant mediational effects were found for adolescents. These findings highlight potential explanations for associations between BP traits and reactive and proactive aggression in young adults, and indicate that reactive aggression in adult BPs could be decreased by reducing emotional coping, and proactive aggression by reducing avoidant coping. Aggr. Behav. 38:403‐413, 2012. © 2012 Wiley Periodicals, Inc.