Background National Center for Health Statistics (NCHS) data for U.S. women have shown a steady decline in breast cancer mortality rates since 1989. Purpose To analyze U.S. breast cancer mortality rates by age decade in women aged 20-79 years and in women aged 20-39 years and women aged 40-69 years. Materials and Methods The authors conducted a retrospective analysis of (a) female breast cancer mortality rates from NCHS data for 1969-2017 for all races and by race and (b) age- and delay-adjusted invasive breast cancer incidence rates from the Surveillance, Epidemiology, and End Results program. Joinpoint analysis was used to determine trends in breast cancer mortality, invasive breast cancer incidence, and distant-stage (metastatic) breast cancer incidence rates. Results Between 1989 and 2010, breast cancer mortality rates decreased by 1.5%-3.4% per year for each age decade from 20 to 79 years (P < .001 for each). After 2010, breast cancer mortality rates continued to decline by 1.2%-2.2% per year in women in each age decade from 40 to 79 years (P < .001 for each) but stopped declining in women younger than 40 years. After 2010, breast cancer mortality rates demonstrated nonsignificant increases of 2.8% per year in women aged 20-29 years (P = .11) and 0.3% per year in women aged 30-39 years (P = .70), results attributable primarily to changes in mortality rates in White women. A contributing factor is that distant-stage breast cancer incidence rates increased by more than 4% per year after the year 2000 in women aged 20-39 years. Conclusion Female breast cancer mortality rates have stopped declining in women younger than 40 years, ending a trend that existed from 1987 to 2010. Conversely, mortality rates have continued to decline in women aged 40-79 years. Rapidly rising distant-stage breast cancer rates have likely contributed to ending the decline in mortality rates in women younger than 40 years. © RSNA, 2021 Online supplemental material is available for this article.