BACKGROUND: The mental health burden among adolescents has been increasing, impacting individuals even before formal diagnosis of common mental disorders. Although personality traits, as key indicators of mental health conditions, play a crucial role in the development of mental disorders, there is a gap regarding the trait-to-symptom pathways and similarities and differences between adolescents and young adults. METHODS: A total of 860 adolescents and 1751 young adults participated in this study. The Chinese Big Five Personality Inventory assessed the Big Five traits, while depression and anxiety symptoms were measured using the Patient Health Questionnaire-9 and the Generalized Anxiety Disorder-7, respectively. Network analysis computed the bridging centrality of Big Five traits and elucidated trait-to-symptom pathways. Furthermore, network comparison was applied to compare network structure between adolescents and young adults. RESULTS: In both age groups, neuroticism exhibited a transdiagnostic activating effect on depression and anxiety. Conscientiousness demonstrated the strongest protective effect against depression, whereas agreeableness was most protective against anxiety. In both samples, neuroticism primarily influenced symptoms associated with negative emotions and thoughts. Comparatively, extraversion exhibited a significant increasing protective effect against depression throughout adolescence, while neuroticism increasingly activated anxiety symptoms. CONCLUSIONS: This study highlights the potential to utilize personality traits for early detection and precise intervention in adolescent populations, providing actionable insights. By identifying the level of neuroticism, we can effectively detect high-risk adolescent individuals prior to formal diagnosis. By delineating the neuroticism-to-symptom pathways, we can implement targeted intervention on their pathological interactions.