Background: Suboptimal treatment delays is known to impact prognosis of patients with cancer but optimal timing in specific subgroups remains poorly studied. This study aimed to analyze treatment delays in young women treated for a breast cancer (BC) on and its impact on their prognosis using French Nationwide Data. Methods: Using the CAREPAT-YBC Cohort based on the French National Healthcare System Database, we analyzed disease-free survival (DFS) in 22,093 young women (18-45 years) who underwent either surgerychemotherapy-radiotherapy pathway (adjuvant setting, 15,433 patients) or chemotherapy-surgeryradiotherapy pathway (neoadjuvant setting, 6660 patients), according to delays between the different pathways. Results: For the adjuvant chemotherapy-radiotherapy interval, the best timing was 17-31 days with increased risk above this delay. For the neoadjuvant setting, the optimal neoadjuvant chemotherapy-surgery interval was 17-31 days, while <= 15 days (HR 1.44, 95%CI 1.21-1.71) or >= 62 days (HR 2.07, 95%CI 1.36-3.15) showed poorer prognosis. Combining best timing into an "optimal pathway" was associated with respectively a 1.2-fold decreased risk for recurrence or mortality. Conclusion: Optimizing treatment intervals enhance BC survival in younger age.