Scholars agree that young men carry out most acts of political violence. Still, there is no consensus on the link between relatively large youth cohorts and the onset of violent, armed intra-state conflicts. In this paper, we examine the effect of youth bulge, a measure of the relative abundance of youth in a country, on the onset of two different types of civil wars—ethnic and non-ethnic wars. Building on and extending three datasets used by other scholars, we theoretically argue and empirically substantiate that, as a result of the negative effects of youth bulge on the economic conditions of the youth cohorts in the country, youth bulge affects the onset of non-ethnic wars, but not the onset of ethnic wars. Possible implications and directions for further research are then suggested.