Background: Approximately one-third of young people in the UK have suffered intimate partner violence and abuse (IPVA) on reaching adulthood. We need interventions to prevent IPVA in this population, but there is a lack of evidence on who is at greatest risk. Methods: We analysed questionnaire data from 3,279 participants of the Avon Longitudinal Study of Parents and Children population-based birth cohort. We estimated the prevalence of IPVA victimisation and perpetration by age 21, by sex, demographic, parenting, mental health, externalising behaviour (e.g. smoking), educational, employment, and adverse childhood factors. Results: Overall, 29% of males and 41% of females reported IPVA victimisation, with 20% and 25% reporting perpetration, respectively (16% and 22% both). The most common sub-type was emotional, followed by physical, then sexual. History of self-harm, anti-social behaviour, cannabis or illicit (non-cannabis) drug use among boys and girls was associated with a two-fold increase in likelihood of IPVA (victimisation or perpetration). Males reporting risky sexual behaviour, sexual abuse (not by an intimate partner), or witnessing domestic violence, and females reporting sexual minority status in adolescence were also twice as likely to experience IPVA. Extreme parental monitoring during adolescence was associated with a reduced risk of IPVA in males and females, as was not being in education, employment, or training for young adult men. Conclusions: A range of demographic, mental health, and behavioural factors were associated with increased prevalence of IPVA victimisation or perpetration. Further study of likely complex pathways from these factors to IPVA, to inform primary prevention, is needed.