Aging is associated with tissue-level changesin cellular composition that are correlated withincreased susceptibility to disease. Aging humanmammary tissue shows skewed progenitor cell po-tency, resulting in diminished tumor-suppressivecelltypes and the accumulation ofdefective epithelialprogenitors. Quantitative characterization of theseage-emergent human cell subpopulations is lacking,impeding our understanding of the relationship be-tween age and cancer susceptibility. We conductedsingle-cell resolution proteomic phenotyping ofhealthy breast epithelia from 57 women, aged 16?91years, using mass cytometry. Remarkable hetero-geneity was quantified within the two mammaryepithelial lineages. Population partitioning identifiedasubsetofaberrantbasal-likeluminalcellsthataccu-mulate with age and originate from age-alteredprogenitors. Quantification of age-emergent pheno-types enabled robust classification of breast tissuesby age in healthy women. This high-resolution map-ping highlighted specific epithelial subpopulationsthat change with age in a manner consistent withincreased susceptibility to breast cancer.