Epidemiologic studies of breast cancer have identified a number of factors that alter breast cancer risk, many of which have different effects in younger and older women. Incidence rates of breast cancer increase rapidly until around the age of menopause or age 50, after which they increase more gradually [1]. Among women younger than 40 years of age, Blacks are at 10-40 percent higher risk than Whites, an excess that is not explained by differences in risk factors [2]. Risk is greater among older white women.

An early age at menarche has been related to an increased risk of breast cancer in both younger and older women [3]. Pregnancy increases breast cancer risk in the short-term, particularly immediately after delivery or for an age at first birth of 30 or later, but reduces risk later on [4]. Duration of lactation has been associated with a reduced risk of breast cancer, particularly in premenopausal women [5]. Oral contraceptive use is associated with a small increase in breast cancer risk among current users and those who stopped within the previous 10 years, but not among those who stopped longer ago [6]. Obesity is associated with a reduction in breast cancer risk in premenopausal women, but is related to increased risk in post-menopausal women [3]. Risk associated with a first-degree family history of breast cancer is greatest in those with a young age at diagnosis, as is risk associated with atypical hyperplasia [3]. There is no evidence that the modest association between alcohol consumption and breast cancer risk is stronger in younger women at diagnosis than in older women [3].

Epidemiologic studies suggest that women diagnosed with breast cancer at younger ages have poorer survival compared to older women with similarly-staged tumors [7]. Cancers diagnosed in younger women are more often hormone-receptor negative, have higher S-phase fractions, and present with higher grade. In addition, some studies have shown abnormal p53 expression in most tumors from women under age 35 [7].