It has frequently been suggested that exceptionally large youth cohorts, the so-called “youth bulges,” make countries more susceptible to political violence. Within two prominent theoretical frameworks in the study of civil war, youth bulges are argued to potentially increase both opportunities and motives for political violence. This claim is empirically tested in a time-series cross-national statistical model for internal armed conflict for the period 1950–2000, and for event data for terrorism and rioting for the years 1984–1995. The expectation that youth bulges should increase the risk of political violence receives robust support for all three forms of violence. The results are consistent both with an expectation that youth bulges provide greater opportunities for violence through the abundant supply of youths with low opportunity costs, and with an expectation that stronger motives for violence may arise as youth bulges are more likely to experience institutional crowding, in particular unemployment. Some contextual factors have been suggested to potentially enhance the effect of youth bulges. In an empirical test of these propositions, the study suggests that youth bulges are particularly associated with an increasing risk of internal armed conflict in starkly autocratic regimes, but a similar effect is also found for highly democratic countries. The interaction of youth bulges with economic decline and expansion in higher education appear to increase the risk of terrorism but not of rioting. Recent studies in economic demography find that when fertility is sharply decreasing, causing lower dependency ratios, large youth cohorts entering the labor market may lead to economic boosts. This study finds some empirical evidence complementing these results, indicating that the effect of youth bulges on political violence may decline along with reduced dependency ratios.