Abstract: Objective: Non-Suicidal Self-Injury (NSSI) has emerged as an important mental-health concern. However, epidemiological features like age of onset of NSSI have remained understudied. Therefore, the current study investigated the distribution of age of onset of NSSI in pooled sample of Dutch-speaking adolescents and emerging adults using event history analysis. Method: Eleven datasets measuring age at first NSSI in community and clinical participants collected by researchers in the Dutch-speaking part of Belgium were pooled together. The final dataset consisted of 1973 community males, 1901 community females, and 505 clinical females. Discrete-time event history analysis was used to model the effect of gender and psychiatric disorders on the age of onset of NSSI. Results: Twenty-one percent of adolescents from the community samples engaged in at least one episode of NSSI by the age of 25 years. Irrespective of the type of sample (community or psychiatric disorder), the probability of age of onset peaked around the age of 14-15 years. A second peak was observed around the age of 20 and 24 years in the community and psychiatric samples respectively. Conclusions: Psychosocial interventions for prevention of NSSI should not only target adolescence through school mental health programs but also target emerging adults at the university level. (C) 2017 Elsevier Inc. All rights reserved.