This study drew on four cycles of longitudinal data from the Canadian National Longitudinal Survey of Children and Youth to examine the academic and behavioural trajectories of youth between 10 and 15 years of age as a function of maternal age at childbearing. The analyses controlled for several family characteristics and examined the mediating effects of three family functioning variables (maternal depression, and nurturing and rejecting parenting behaviours). Maternal age was related to academic competency in math (standardized Math scores), externalizing disorders (Property Offences, Hyperactivity-Inattention), and internalizing disorders (Anxiety-Emotional Disorder). After accounting for family characteristics, the children of the young and older teen mothers retained their disadvantage in Math scores and Property Offences, respectively, whereas the children of older mothers lost the advantages that they had exhibited at age 10 (for Property Offences, the benefits were mediated through family functioning). These results have implications for future research and for the development of policy and programming targeting the healthy development of youth.