Early diagnosis of Autism Spectrum Disorder is considered best practice, increasing access to early intervention. Yet, many children are diagnosed after 3-years. The current study investigated the school age outcomes of children who received an early and later diagnosis of ASD. The cognitive and behavioural outcomes of children diagnosed early (n = 48), were compared to children diagnosed after 3-years (n = 37). Children diagnosed early accessed more intervention, demonstrated better verbal and overall cognition at school age, were more likely to attend mainstream school and required less ongoing support than children diagnosed later. Behavioural differences were not found between groups. Earlier diagnosis is important and is likely to promote more positive outcomes at school age due to increased opportunity for EI.