Eating disorders (EDs) are typically diagnosed in the later stages of puberty, but risk factors for EDs are present in pre-pubertal children. This longitudinal, population-based birth cohort study aimed to examine prospective predictors of eating disorder symptoms in 15-year-olds. Specifically, we sought to test an adapted dual pathway model of disordered eating aetiology in this group. Participants in the Gateshead Millennium Study birth cohort (n = 326; 187 girls and 175 boys) completed self-report questionnaires assessing eating disorder symptoms and risk factors at ages 7, 9, 12, and 15 years. Measures included body image, depressive symptoms, and pubertal development; we also measured BMI at each age. The data were fitted to a model of eating disorder symptom development to help us understand the role of puberty, adiposity, body dissatisfaction, depressive symptoms, and previous eating disorder symptoms. We found that previous eating disorder symptoms were the strongest predictor of eating disorder symptoms at 15, and that depressive symptoms prospectively predicted eating disorder symptoms in girls. We furthermore found that depressive symptoms at 12 partially mediated the relationship between body dissatisfaction at 12 and eating disorder symptoms at 15 in girls. Pubertal development predicted concurrent eating disorder symptoms at 12 in girls but not boys. Overall, our findings support the roles of pubertal development, body dissatisfaction, and depressive symptoms in the pathogenesis of eating disorder symptoms across puberty. They add to growing evidence that there are different pathways to eating disorder symptoms in girls and boys.