Recent advances and debates surrounding general and developmental as well as static and dynamic theories of crime can be traced to the 1986 National Academy of Science's Report on criminal careers and the discussion it generated. A key point of contention has been regarding the interpretation of the age-crime curve. According to Gottfredson and Hirschi (1986), the decline in the age-crime curve in early adulthood reflects decreasing individual offending frequency (?) after the peak. Blumstein et al. (1986) claimed that the decline in the aggregate age-crime curve also could be attributable to the termination of criminal careers, and the average value of l could stay constant (or increase with age) for those offenders who remain active after that peak. Using data from the Criminal Career and Life Course Study-including information on criminal convictions across 60 years of almost 5,000 persons convicted in the Netherlands-and applying a two-part growth model that explicitly distinguishes between participation and frequency, the study outlined in this article assessed the participation-frequency debate. Results suggest that the decline in the age-crime curve in early adulthood reflects both decreasing individual offending participation and frequency after the peak, that the probabilities of participation and frequency are significantly related at the individual level, and that sex and marriage influence both participation and frequency.