Aim: Research is required to study the relationship between age of autism spectrum disorder (ASD) diagnosis and the presence of comorbidities. Method: The Gastrointestinal Symptom Inventory, Autism Spectrum Disorder-Comorbid for Children, Behavior Problem Inventory-Short Form and Social Communication Questionnaire were completed by parents of 129 children and adolescents with a diagnosis of ASD. Results: Results revealed significant relationships between the age of ASD diagnosis, the presence of comorbidities and intellectual disability. Significant correlations were found between the age of ASD diagnosis and self-injurious and stereotyped behavior. Comorbid psychopathology significantly predicted the presence of GI symptoms. In addition, the relationship between comorbid psychopathology and challenging behavior in this study was reported as bi-directional as both comorbidities predicted one another in the sample. Conclusion: Future research needs to consider the role of comorbidities in relation to ASD diagnosis.