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The age-crime curve, increasing to a peak in the teenage years and then decreasing, is well-known. Less well-known is that it seems to reflect variations in prevalence (the proportion of persons who are offenders) rather than incidence (the rate of offending by offenders). Age-crime curves for individuals do not resemble the aggregate curve since incidence does not change consistently between the onset and the termination of criminal careers. This has major implications for criminal justice policy since the greatest residual length of criminal careers, and hence the greatest potential incapacitative effect, may be between ages thirty and forty, not at the peak age. Different types of offenses peak at different ages; this probably reflects crime switching rather than the replacement of one group of offenders by another. There is little specialization in offending, but specialization does increase with age. Age effects need to be separated from period and cohort effects. The age-crime curve probably reflects decreasing parental controls, a peaking of peer influence in the teenage years, and then increasing family and community controls with age.