Background: African American (AA) women have higher incidence of aggressive, young-onset (<40 years) breast cancers. Young- and older-onset disease may have distinct tumor biologies and etiologies; however, studies investigating age differences among AA women have been rare and generally underpowered. Methods: We examined tumor characteristics and breast cancer risk factors associated with premenopausal young (<40) vs. older (40) AA women's breast cancer in the African American Breast Cancer Epidemiology and Risk Consortium (2,008 cases and 5,144 controls). Unconditional logistic regression models assessed heterogeneity of tumor biology and risk factor associations by age, overall, and by estrogen receptor status. Results: Premenopausal AA women <40 years had higher frequency of poorer-prognosis tumor characteristics compared with older women, including negative estrogen and progesterone receptor status, triple-negative subtype, higher grade, higher stage, and larger tumors. Adiposity (i.e., waist-to-hip ratio) and family history of breast cancer were more strongly associated with young-onset disease [case–control OR ¼ 1.46, 95% confidence interval (CI) ¼ 1.04–2.05; OR ¼ 3.10, 95% CI ¼ 2.08–4.63, respectively] compared with older-onset disease (OR ¼ 1.11, 95% CI ¼ 0.91–1.35; OR ¼ 1.57, 95% CI ¼ 1.26–1.94). Breastfeeding showed a slight inverse risk association among young women (OR ¼ 0.70, 95% CI ¼ 0.43–1.16). Oral contraceptive use was associated with increased risk regardless of age. Considering various cutoff points for young age (<40, <45, <50), age-related heterogeneity was greatest when <40 was used. Conclusions: Among premenopausal AA women, diagnosis before age 40 is associated with more aggressive breast tumor biology and some etiologic differences. Impact: Modifiable risk factors including breastfeeding, adiposity, and oral contraceptive use may be important targets for mitigating harms of young-onset breast cancer.
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