A 56-yeаr-оld wоmаn presents tо the office to discuss her mаmmography results that show overall dense breasts with a spiculated mass and clustered microcalcifications on the right. Her family history is negative for breast, endometrial, and ovarian cancers. She is a former flight attendant. Since retirement, she has started a strict Mediterranean diet but does not do regular physical activity/exercise. She is a mother of 2 and breastfed both babies for 18 months. Past surgical history is significant for breast augmentation with implants and tubal ligation. On physical examination, her BMI is 25 kg/m2; breast exam shows no skin or nipple changes and no palpable masses. What is the most significant risk factor for breast cancer in this patient?