Introduction: NSSI (non-suicidal self-injury) is an important public health issue, with high prevalence and associations with future mental illness and suicide. To date, no large single study has tested age and gender effects on NSSI and their interplay with psychological distress. Method: NSSI and psychological distress were ascertained by questionnaire in a community study of 2368 young people aged 14-25; proportions at each age and of each gender were approximately equal. Results: There was a significant quadratic interaction between age and gender on last month NSSI prevalence (p = 0.025): NSSI was more common in females ages 16-19, but there were no significant gender differences at younger/older ages. General distress partially mediated the effects of age and gender on NSSI. The association between general distress and NSSI was not significantly moderated by age, gender nor their interactions. Conclusions: Gender difference in NSSI is not a static gap, but evolves across time, widening in mid-adolescence and disappearing by early adulthood. Part of the reason for that gender gap being present at those ages is the increase levels of distress in young women at those ages. There was no evidence that the effects of general distress on NSSI differed by age/gender.