Purpose: An official autism diagnosis is required to access timely intervention and is associated with better long-term wellbeing and mental health. Certain demographic characteristics, such as being female or a racially or ethnically minoritized youth, have been associated with significant diagnostic lag. However, it remains unclear how assigned sex, race, and ethnicity interact with each other in predicting the prevalence and age of autism diagnosis. Methods: To examine the interactions between assigned sex, race, and ethnicity, we used data from the National Survey of Children's Health (NSCH; 2016 > 2021). Results: One in 38 children had an autism diagnosis and 3.8 males were diagnosed per 1 female. Hierarchical linear regressions yielded diagnostic delays in some females, particularly those who were non-Hispanic white, Black, and Asian. Ethnic and racial minority children had significantly earlier diagnoses than white and non-Hispanic children when not accounting for sex. Conclusion: This study demonstrates slight increases in reported autism prevalence, a diagnostic lag in some autistic females that was strongly associated with ethnicity, and earlier diagnoses in racial and ethnic minority youth, a finding that may be explained by factors associated with phenotypic differences. This study has important implications for the diagnosis of minority autistic youth, particularly females and females who are non-Hispanic, who may experience a greater propensity for diagnostic delays.