Is it rare for young women from 15 - 20 to develop breast cancer?

Yes: Breast cancer is very rare before the age of 30. There are families that have a hereditary breast cancer gene and cancer can occur at younger ages in these families.

Small but not zero: It would be rare to have breast cancer in a teenage female without a strong family histroy, but if there is a concern, bring it up to your primary care physician. It is recommended for women to get mammograms starting at age 35-40 or 10 years before any relative was diagnosed with breast cancer. Therefore, if your aunt got breast cancer at 40, you should start mammograms at 30. Ask your doctor.

< 1 in a Million: The incidence of breast cancer increases with age, occurring very rarely in one's 20's, sporadically in the 30's, and then increasing with each decade of life. An adolescent with a breast lump still requires physician evaluation, but a benign tumor called a fibroadenoma is far more likely.