AIMS: Clinical trials of post-mastectomy radiotherapy (PMRT) for early invasive breast cancer (EIBC) have included few older women. This study examined whether the association between overall survival or breast cancer-specific survival (BCSS) and receipt of PMRT for EIBC altered with age. MATERIALS AND METHODS: The study used patient-level linked cancer registration, routine hospital and radiotherapy data for England and Wales. It included 31 243 women aged ≥50 years diagnosed between 2014 and 2018 with low- (T1-2N0), intermediate- (T3N0/T1-2N1) or high-risk (T1-2N2/T3N1-2) EIBC who received a mastectomy within 12 months from diagnosis. Patterns of survival were analysed using a landmark approach. Associations between overall survival/BCSS and PMRT in each risk group were analysed with flexible parametric survival models, which included patient and tumour factors; whether the association between PMRT and overall survival/BCSS varied by age was assessed using interaction terms. RESULTS: Among 4711 women with high-risk EIBC, 86% had PMRT. Five-year overall survival was 70.5% and BCSS was 79.3%. Receipt of PMRT was associated with improved overall survival [adjusted hazard ratio (aHR) 0.75, 95% confidence interval 0.64-0.87] and BCSS (aHR 0.78, 95% confidence interval 0.65-0.95) compared with women who did not have PMRT; associations did not vary by age (overall survival, P-value for interaction term = 0.141; BCSS, P = 0.077). Among 10 814 women with intermediate-risk EIBC, 59% had PMRT; 5-year overall survival was 78.4% and BCSS was 88.0%. No association was found between overall survival (aHR 1.01, 95% confidence interval 0.92-1.11) or BCSS (aHR 1.16, 95% confidence interval 1.01-1.32) and PMRT. There was statistical evidence of a small change in the association with age for overall survival (P = 0.007), although differences in relative survival were minimal, but not for BCSS (P = 0.362). CONCLUSIONS: The association between PMRT and overall survival/BCSS does not appear to be modified by age among women with high- or intermediate-risk EIBC and, thus, treatment recommendations should not be modified on the basis of age alone.