Breast cancer incidence increases with age, but the molecular features of cancers arising in older patients remain poorly characterized. We examined the age-associated differences in clinicopathologic characteristics, transcriptional pathways, and tumour microenvironment signatures using clinical and gene-expression data from three large cohorts. Patients were grouped as younger than 45, 45–64, or 65 years and older; the oldest group was further divided into 65–74, 75–84, and 85 years and older. Among the included 6323 patients, those younger than 45 had a higher proportion of hormone-receptor negative subtypes and grade 3 tumours with enrichment of proliferation-related pathways. Comparisons between patients aged 65–74 and 75–84 did not show a consistent shift towards markers of less aggressive tumour biology. Instead, metabolic-related differences such as in oxidative phosphorylation, glycolysis, and mechanistic target of rapamycin complex 1 signalling were noted. Immune cell signature differences were most evident in younger hormone-receptor positive tumours. These findings overall support evaluating tumour biology instead of chronological age in guiding future treatment strategies and research.