Age of onset is a marker of clinically relevant subtypes in various medical and psychiatric disorders. Past research has also reported that age of onset in generalized anxiety disorder (GAD) is clinically significant; but, in research to date, arbitrary cut-off ages have been used. In the present study, admixture analysis was used to determine the best fitting model for age of onset distribution in GAD. Data were derived from 459 adults with a diagnosis of GAD who took part in the Netherlands Study of Depression and Anxiety (NESDA). Associations between age of onset subtypes, identified by admixture analysis, and sociodemographic, clinical, and vulnerability factors were examined using univariate tests and multivariate logistic regression analyses. Two age of onset distributions were identified: an early-onset group (24 years of age and younger) and a late-onset group (greater than 24 years of age). Multivariate analysis revealed that early-onset GAD was associated with female gender (OR 2.1 (95%CI 1.4–3.2)), higher education (OR 1.1 (95%CI 1.0–1.2)), and higher neuroticism (OR 1.4 (95%CI 1.1–1.7)), while late-onset GAD was associated with physical illnesses (OR 1.3 (95%CI 1.1–1.7)). Study limitations include the possibility of recall bias given that age of onset was assessed retrospectively, and an inability to detect a possible very-late-onset GAD subtype. Collectively, the results of the study indicate that GAD is characterized by a bimodal age of onset distribution with an objectively determined early cut-off at 24 years of age. Early-onset GAD is associated with unique factors that may contribute to its aetiology; but, it does not constitute a more severe subtype compared to late-onset GAD. Future research should use 24 years of age as the cut-off for early-onset GAD to when examining the clinical relevance of age of onset for treatment efficacy and illness course.