Phenotypic elements of autism spectrum disorder can be masked by attention-deficit/hyperactivity disorder symptoms, potentially leading to a misdiagnosis or delaying an autism spectrum disorder diagnosis. This study explored differences in the age of autism spectrum disorder diagnosis between participants with previously diagnosed attention-deficit/hyperactivity disorder versus autism spectrum disorder-only respondents. Children and adolescents, but not adults, initially diagnosed with attention-deficit/hyperactivity disorder received an autism spectrum disorder diagnosis an average of 1.8 years later than autism spectrum disorder--only children, although the findings regarding the adult sample should be interpreted with caution. Gender differences were also explored, revealing that the delay in receiving an autism diagnosis was 1.5 years in boys and 2.6 years in girls with pre-existing attention-deficit/hyperactivity disorder, compared with boys and girls without prior attention-deficit/hyperactivity disorder. No significant gender differences were observed in the adult sample. We argue that overlapping symptoms between autism spectrum disorder and attention-deficit/hyperactivity disorder might delay a formal diagnosis of autism either by leading to a misdiagnosis of attention-deficit/hyperactivity disorder or by making it difficult to identify the presence of co-occurring autism spectrum disorder conditions once an initial diagnosis of attention-deficit/hyperactivity disorder has been obtained. Current findings highlight the need to recruit multidimensional and multidisciplinary screening procedures to assess for potential emerging autism spectrum disorder hallmarks in children and adolescents diagnosed or presenting with attention-deficit/hyperactivity disorder symptoms.