Symptoms of autism spectrum disorders may appear as early as 6 months, but parent concern, which can precipitate evaluation, often lags significantly. The presence of typical or atypical older siblings can change parents’ sensitivity to departures from typical development. This study investigated type and age of parent’s first concerns in toddlers with autism spectrum disorder, prior to diagnosis. Participants had (1) at least one older sibling with autism spectrum disorder (Sibs-ASD); (2) only typically developing older siblings (Sibs-TD), or (3) were only/oldest (No-Sibs). Specific autism spectrum disorder diagnoses and symptom severity were similar among groups. Developmentally, No-Sibs showed the largest delays, followed by Sibs-TD, followed by Sibs-ASD. Mean age of first concern was 16 months for No-Sibs, 14 months for Sibs-TD, and 10 months for Sibs-ASD. Age of first concern differed significantly by group, even after controlling for mother’s age and education. Concern about language was prevalent in all groups. Thus, the presence of an older child with typical or, especially, atypical development was associated with earlier concerns for the affected child, despite milder developmental delays. These findings underscore the importance of encouraging parents to report concerns to pediatricians, routine standardized screening for autism spectrum disorder, and the need for pediatrician vigilance, especially for only or oldest children.