A relationship between trends in the age structure of the population and homicide rates has been systematically observed only in the United States since World War II. This study analyzes data from five nations over 70 years to examine the generalizability of this relationship. Using two time-series techniques, we find that changes in the proportion of young males in a population do not exert consistent effects on homicide rates. There is no evidence of an age effect in Scotland and Japan, and strong evidence of a postwar age effect only in the United States and Italy. Based on similarities in the character of homicide in the United States and Italy, we suggest a set of conditions under which an age effect is most likely to obtain.