There are a host of complex and interlinked psychological, social and biological factors involved in the development of problem gambling (PG). While existing research, which is predominantly cross-sectional, shows that emerging adulthood is a critical period for PG, the early risk factors for PG are currently unknown. Here, we recruited a sample of 156 early adolescents with no history of PG (mean age 12.6 years) and longitudinally followed them up into late adolescence (mean age 18.9 years) to investigate the role of sex, risk-taking behaviour and changes in temperament and psychiatric symptoms in the evolution of risky gambling behaviour. There were sex-independent effects of temperament and risk-taking behaviour, with greater developmental increases in temperamental frustration (i.e. negative affectivity), greater developmental decreases in temperamental attention (i.e. effortful control) and greater involvement in risky behaviours, such as alcohol use, predicting greater likelihood of being in the risky gambling group. In addition, there were sex-dependent effects whereby higher levels of baseline aggression in females and lower levels of the same in males were more predictive of risky gambling. These findings highlight how sex-dependent and independent factors across the early- to mid-adolescent period influence the occurrence of gambling later in life.