Background: The diagnosis of autism spectrum disorder (ASD) is an exceptionally crucial and sensitive topic in the field of mental health. Despite the importance of early diagnosis, there is often a delay in recognizing autism, especially in the particularly sensitive age group of adolescent boys. The main consequences of this delay are a lack of support, incorrect pharmacological treatment, psychological bullying, family strain, decline in social interaction, and reinforcement of ASD traits. This systematic literature review aims to study the phenomenon of delayed diagnosis of autism in boys during adolescence or young adulthood. Methods: Eligible studies were extracted from PubMed, Google Scholar, and Scopus databases up to August 30, 2023. The participants included only boys, adolescents aged 10–18 years. Results: A total of 14 eligible studies and 5.185 participants were included. The primary reasons for delayed ASD diagnosis concerned prior misdiagnosis, comorbidities, social stigma, socio-economic factors, lack of specialized education and parental awareness, diagnostic reclassification, and absence of ASD characteristics. Conclusions: Substantial delays are recorded between the initial concern and the final assessment of boys, resulting in diagnoses occurring during adolescence or adulthood.