[Correction Notice: An Erratum for this article was reported in Vol 56(7) of Journal of the American Academy of Child & Adolescent Psychiatry (see record 2017-28619-020). In the original article, the Medical Research Council was incorrectly credited as the sponsor. Open access was funded by the Seaver Autism Center for Research and Treatment and the Icahn School of Medicine at Mount Sinai.] Objective: Advanced paternal age (APA) at conception has been linked with autism and schizophrenia in offspring, neurodevelopmental disorders that affect social functioning. The current study explored the effects of paternal age on social development in the general population. Method: We used multilevel growth modeling to investigate APA effects on socioemotional development from early childhood until adolescence, as measured by the Strengths and Difficulties Questionnaire (SDQ) in the Twins Early Development Study (TEDS) sample. We also investigated genetic and environmental underpinnings of the paternal age effects on development, using the Additive genetics, Common environment, unique Environment (ACE) and gene-environment (GxE) models. Results: In the general population, both very young and advanced paternal ages were associated with altered trajectory of social development (intercept: p = .01; slope: p = .03). No other behavioral domain was affected by either young or advanced age at fatherhood, suggesting specificity of paternal age effects. Increased importance of genetic factors in social development was recorded in the offspring of older but not very young fathers, suggesting distinct underpinnings of the paternal age effects at these two extremes. Conclusion: Our findings highlight that the APA-related deficits that lead to autism and schizophrenia are likely continuously distributed in the population.