Introduction: Our study aimed to compare the sociodemographic, diagnostic, clinical, and self-report scale data of adults diagnosed with ADHD in childhood/adolescence versus adulthood and to identify risk factors associated with delayed/missed diagnosis for ADHD. Method: Sociodemographic, clinical, and diagnostic data of 214 adults with ADHD, followed at the Adult Neurodevelopmental Disorders Clinic, Selçuk University, between January 2022 and January 2024, were analyzed. The diagnostic evaluations were made using the Structured Clinical Interview for DSM-5 Clinician Version. Clinical data included age, gender, years of education, alcohol/substance use, diagnosis age, and current medication use for ADHD. Data collected from self-report scales included both ADHD-related measurements and comorbidity-related measurements. Results: The findings revealed that only 34.4% (n = 74) of sample received a formal ADHD diagnosis during childhood/adolescence. Adults diagnosed with ADHD in adulthood were older and had higher education levels, more severe ADHD symptoms, and increased maladaptive daydreaming (MD) scores, compared to those diagnosed in childhood or adolescence. Logistic regression analysis indicated that the severity of MD was associated with being in the group diagnosed with ADHD in adulthood while controlling for other significant parameters from bivariate analyses, such as age, years of education, and current medication use for ADHD. Finally, analyses conducted separately in both groups showed that: (1) increased MD severity was a predictor of higher ADHD symptoms in those diagnosed in adulthood, but not in those diagnosed in childhood/adolescence, and (2) MD severity had a stronger correlation with ADHD symptoms, the number of comorbid psychiatric disorders, and symptoms of excessive mind wandering, depression, and anxiety in those diagnosed in adulthood compared to those diagnosed in childhood/adolescence. Conclusion: MD may delay ADHD diagnosis until adulthood by masking and compensating ADHD symptoms and delay in referral to mental health professionals, and it might also be a predictive symptom for recognizing ADHD in adults who have never been diagnosed; however, longitudinal studies are needed to confirm this.