Criminologists have traditionally considered adult-onset offending to be a rare phenomenon (Eggleston & Laub 2002). Consequently, little criminological theory, research or policy has focused on adult-onset offending. However, an emerging body of research suggests that a substantial number of offenders have their first contact with the criminal justice system at 18 years of age or older (Block et al. 2010; Delisi & Piquero 2011; Eggleston & Laub 2002; Kratzer & Hodgins 1999; McGee & Farrington 2010). These findings have generated increasing interest in adult-onset offenders in recent years. Despite this, the nature of adult-onset offending is still not well understood (Zara & Farrington 2013). The limited research that has been conducted suggests that adult-onset offenders may perpetrate different types of offences to early-onset offenders (McGee & Farrington 2010) and their offending may be associated with different risk factors (Zara & Farrington 2013). Given these findings, some have questioned whether the same traditional criminal justice responses are appropriate for adult-onset and early-onset offenders (McGee & Farrington 2010; Zara & Farrington 2013). In the current study, the extent, nature and costs of adult-onset offending will be examined in a Queensland population-based offender cohort (1983/1984 Queensland Longitudinal Cohort; 83/84 QLD). Adult-onset offenders will be compared with early-onset offenders to determine if and how their offending profiles differ. In addition, the appropriateness and cost-effectiveness of current criminal justice responses to adult-onset offending will be examined.