Later autism diagnosis is associated with increased mental health risks. Understanding disparities in diagnostic timing is important to reduce psychiatric burden for autistic people. One characteristic associated with later autism diagnosis is female sex assigned at birth. However, literature to date does not characterize, differentiate, or account for gender identity beyond assigned sex at birth. Gender diversity may be more common in autistic relative to neurotypical people, and autism is proportionally overrepresented in gender-diverse populations. We examined age at autism diagnosis by assigned sex at birth, gender identity, and gender diversity (gender-diverse vs cisgender) status, separately. Three independent cohorts representing different ascertainments were examined: a research-recruited academic medical center sample (N = 193; 8.0-18.0 years); a clinic-based sample (N = 1550; 1.3-25.4 years); and a community-enriched sample (N = 244, 18.2-30.0 years). The clinic-based and community-enriched samples revealed disparities in diagnostic timing: people assigned female at birth, people of female gender, and gender-diverse people were diagnosed with autism significantly later than persons assigned male at birth, persons of male gender, and cisgender persons, respectively. Birth-sex, gender identity, and gender diversity may each uniquely relate to disparities in autism diagnostic timing. The influence of ascertainment strategies, particularly in studies examining assigned sex at birth or gender identity, should be considered.