There is disagreement in the literature as to whether there are any true adult-onset offenders. The aim of this study is to investigate the prevalence and correlates of adult-onset offenders in a contemporary British general population cohort consisting of 739 individuals aged between 18 and 25 years. Sixteen percent of participants reported offending for the first time after the age of 18. It is concluded that adult-onset exists and deserves to be studied further. Adult-onset offenders were more likely to report using drugs, associating with deviant peers, and having mental health problems in adulthood than non-offenders. Compared with early-onset offenders, the adult-onset offenders were people with a stronger attachment to school, which may have protected them from the risk of offending in adolescence. It is possible that when that protection was removed in adulthood and they were exposed to negative life events, such as drug use and mental illness, they became involved in crime for the first time.