Purpose: To explore changes in physical and psychosocial function before and after breast cancer by age at diagnosis. Patients and Methods: A total of 122,969 women from the Nurses' Health Study (NHS) and NHS 2, ages 29 to 71 years, who responded to pre- and postfunctional status assessments were included; 1,082 women were diagnosed with breast cancer between 1992 and 1997. Functional status was measured using the Medical Outcomes Study Short Form 36 (SF-36). Mean change in health-related quality of life (HRQoL) scores was computed across categories representing the combination of incident breast cancer (yes or no) and age at diagnosis (≤ 40, 41 to 64, or 65+ years). Results: Compared with women ≤ 40 years without breast cancer, women with breast cancer experienced significant functional declines. Young (age ≤ 40) women who developed breast cancer experienced the largest relative declines in HRQoL (as compared with middle-aged and elderly women) in multiple domains including physical roles (-18.8 v -11.5 and -7.5 points, respectively), bodily pain (-9.0 v -2.7 and -2.7 points), social functioning (-11.3 v -4.3 and -4.4 points) and mental health (-3.1 v 0.0 and +0.4 points). Much of the decline in HRQoL among elderly (age ≥ 65) women with breast cancer was age related. Conclusion: Young women may fare worse than middle-aged or elderly women in both physical and psychosocial dimensions after breast cancer diagnosis. The needs of women facing breast cancer may be better understood within a life stage framework.