The nature of the temporal association between age structure and homicide rates between 1970 and 1999 is examined using U.S. county data. Specifically, the following questions are asked: (a) does the strong temporal association between the relative size of the young population and homicide rates demonstrated at the U.S. national level hold at a disaggregated level and (b) does the association between the proportion young and homicide rates differ by varying social and economic conditions. The results confirm that a positive temporal association between age composition and homicide rates exists within U.S. counties. However, the analysis reveals that other criminogenic forces, such as poor social conditions that produce higher crime rates, can alter the association between the relative size of the young population and homicide rates. The positive effect of the percentage aged 15 to 24 years on homicide rates is generally attenuated when criminogenic conditions exist while that of the percentage aged 25 to 34 years tends to be accentuated under such circumstances. (Contains 3 tables, 5 figures and 13 notes.)