Cell populations in human breast cancers are molecularly and biologically distinct with age.

Aging is associated with increased breast cancer risk, and the oldest and youngest patients have worse outcomes, irrespective of subtype. It is unknown how age affects cells in the breast tumor microenvironment or how they contribute to age-related pathology. Here we discover age-associated differences in cell states in human estrogen receptor-positive and triple-negative breast cancers using analyses of existing bulk and single-cell transcriptomic data. We generate and apply an Age-Specific Program ENrichment (ASPEN) analysis pipeline, revealing age-related changes, including increased tumor cell epithelial-mesenchymal transition and cancer-associated fibroblast inflammatory responses in triple-negative breast cancer. Estrogen receptor-positive breast cancer displays increased ESR1 expression and reduced vascular and immune cell metabolism with age. Cell interactome analysis reveals candidate signaling pathways that drive age-related cell states. Spatial analyses across independent clinical cohorts support the computational findings. This work identifies potential targets for age-adapted therapeutic interventions for breast cancer.