Background Age of onset is an important epidemiological indicator in characterizing disorders subtypes according to demographic, clinical and psychosocial determinants. While investigated in various psychiatric conditions, age of onset and related characteristics in agoraphobia have yet to be examined. In light of the new diagnostic status in the DSM-5 edition of agoraphobia as independent from panic disorder, research on agoraphobia as a stand-alone disorder is needed. Methods Admixture analysis was used to determine the best-fitting model for the observed ages at onset of 507 agoraphobia patients participating in the Netherlands Study of Depression and Anxiety (age range 18-65). Associations between agoraphobia age of onset and different demographic, clinical and psychosocial determinants were examined using multivariate logistic regression analysis. Results Admixture analyses identified two distributions of age of onset, with 27 as the cutoff age (≤ early onset, > late onset). Early onset agoraphobia was only independently associated with family history of anxiety disorders (p<0.01) Limitations Age of onset was assessed retrospectively, and analyses were based on cross-sectional data. Conclusion The best distinguishing age of onset cutoff of agoraphobia was found to be 27. Early onset agoraphobia might constitute of a familial subtype. As opposed to other psychiatric disorders, early onset in agoraphobia does not indicate for increased clinical severity and/or disability.