This study examined whether parent-reported atypical development in their child’s first year was associated with age of diagnosis and age when parents first needed to consult a specialist about their child’s development. It involved 423 children who participated in the Australian Autism Biobank. Most parents retrospectively identified ≥ 1 domain of atypical child development. Atypical development in most domains was associated with an earlier age when parents felt specialist consultation was needed. Atypical development in the “gaze abnormalities”, “lack of response to social stimuli”, and “no social communication” subdomains within the social domain was associated with an earlier age of diagnosis, as was atypical development in the “hypo/hypersensitivity” and “preoccupation with parts of objects” subdomains within the stereotyped/restricted behavior domain.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10803-022-05506-1.