Aim: This study investigated the relationship between sexual debut before 14 years of age and socio-demographics, sexual experience, health, experience of child abuse and behaviour at 18 years of age. Methods: A sample of 3432 Swedish high school seniors completed a survey about sexuality, health and abuse at the age of 18. Results: Early debut was positively correlated with risky behaviours, such as the number of partners, experience of oral and anal sex, health behaviours, such as smoking, drug and alcohol use, and antisocial behaviour, such as being violent, lying, stealing and running away from home. Girls with an early sexual debut had significantly more experience of sexual abuse. Boys with an early sexual debut were more likely to have a weak sense of coherence, low self-esteem and poor mental health, together with experience of sexual abuse, selling sex and physical abuse. A multiple logistic regression model showed that a number of antisocial acts and health behaviours remained significant, but early sexual debut did not increase the risk of psychiatric symptoms, low self-esteem or low sense of coherence at 18 years of age. Conclusion: Early sexual debut was associated with problematic behaviours during later adolescence, and this vulnerability requires attention from parents and healthcare providers.