In this paper, we evaluate behavioral features within three gene groups associated with ID and ASD – ADNP, CHD8, and DYRK1A – with two aims: (1) characterize phenotypes across behavioral domains of anxiety, depression, ADHD, and challenging behavior; and (2) understand whether age and early developmental milestones are associated with later mental health outcomes. This evidence concerns the gene CHD8 and major depressive disorder.