evidence: Breast cancer in young women and people assigned female at birth (AFAB)— also known as early-onset breast cancer — is when you receive a breast cancer diagnosis between the ages of 18 and 45. [...] Breast cancers in people under 45 may also be aggressive and harder to treat based on tumor markers. [...] You can get breast cancer at any age, including when you’re an adolescent or young adult (although this is rare). About 10% of new breast cancer diagnoses are people under 45.