Children with neurodevelopmental disorders tend to have more social difficulties than typically developing children. The aim of the current study was to examine parent and teacher-reported effects of age and gender on social functioning in a large clinical sample of children and adolescents with ADHD, autism, or co-occurring ADHD and autism using a cross-sectional study design. This nationwide clinical sample included 2132 Icelandic children and adolescents (35% girls, 65% boys) aged 5-18 years referred for a neurodevelopmental diagnostic assessment (ADHD and/or autism) in Iceland. Social functioning was measured using the Prosocial behavior and Peer problem subscales on the Strengths and Difficulties Questionnaire (SDQ) completed by parents and teachers. Results revealed that autistic youth and youth with co-occurring ADHD and autism experienced more peer problems and showed less prosocial behavior than youth with ADHD only. According to parents and teachers, girls were found to experience more social difficulties compared to boys. Interaction for age and gender, although only significant for teacher reports, indicated that younger girls with neurodevelopmental disorders experience more peer problems and show less prosocial behavior than older girls. In contrast, boys with neurodevelopmental disorders experience similar issues at all ages. The results suggest different patterns of social difficulties for boys and girls with neurodevelopmental disorders. Future research should examine different developmental pathways of social challenges for boys and girls. Implications for developing and providing clinical interventions appropriate developmental stages are discussed.