This research examined the extent and nature of adult-onset offending and investigated whether adult cautioning could be a viable and cost-effective alternative to current court processing. Adult-onset offending was examined in a Queensland population-based offender cohort (1983–84 Queensland Longitudinal Cohort; n=40,523). The research generated three key findings. First, adult-onset offenders were prevalent, constituting half of all offenders in the cohort. Second, the vast majority of adult-onset offenders were low-rate, less serious offenders. Third, cautioning first-time, low-rate, less serious adult-onset offenders in this cohort would have saved $32.5m in police and court costs. This represents a 23.4 percent cost reduction in processing this group through the criminal justice system and a 4.3 percent reduction in the cost of processing all members of the cohort through the criminal justice system. However, these figures only represent the reduced costs associated with diverting less serious, first-time offences by low-rate adult onset offenders in this single cohort. The cost savings would be substantially more if considered on a cross-sectional basis.