Aims: The aims of the study were to investigate (1) the impact of age at brain insult on functional outcome and (2) the influence of insult and environmental factors on cognitive and behavioural outcomes. Method: The study was a cross-sectional, retrospective observational study, involving 138 children (76 males, 62 females; mean age 13y 1mo, SD 1y 11mo, range 10-16y) with magnetic resonance imaging (MRI) evidence of focal brain insult sustained from the first trimester of pregnancy to adolescence. Children underwent MRI and intellectual, executive, behavioural, and social evaluation. Outcome predictors were insult (lesion location, laterality, and extent, history of seizures, age at insult) and environmental (social risk and family function) factors. Results: Focal insult before the age of 3 years was associated with poorer outcomes than insult after the age of 3 years across all domains. For IQ outcomes, insult characteristics and seizures were highly predictive. For executive and behavioural domains, family function and social risk had the greatest impact. Earlier age at insult predicted poorer social competence. Interpretation: Focal brain insult before age 3 years has devastating consequences for children's development. Findings suggest that greater emphasis should be placed on providing early intervention for children who sustain early focal brain insults.