The use of parent-report screeners for early detection of autism is time- and cost-efficient in clinical settings but their utility may vary by respondent characteristics. This study aimed to examine the degree to which infants’ age and sex impacted parental reports of early behavioral signs of autism captured by the First Years Inventory Version 3.1 (FYIv3.1). The current sample included 6,454 caregivers of infants aged 6 to 16 months recruited through the North Carolina vital records. Using moderated nonlinear factor analysis for each of the seven FYIv3.1, we identified differential item functioning in small to medium effect sizes across 18 out of 69 items, with the majority of biases associated with infants’ age (e.g., object mouthing, walking, pretend, and imitation), while sex-related biases were minimal. This indicates that differential scoring algorithms by infants’ age and more closely spaced monitoring may be needed for these constructs for more accurate identification of autism in infancy.