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“How young is very young for breast cancer” has long been a controversial topic. We first used the X-tile program to determine and proposed that the age of 40 years is a reasonable cutoff value to define “young.” However, the gene expression profiles from the Cancer Genome Atlas proved that young breast cancer might not be a unique biological entity.There is no uniformly adopted cutoff value to define “young patients” with breast cancer. This study was designed to determine an optimal cutoff value, to investigate prognostic factors and to explore gene expression profiles of young female breast cancer.The Surveillance, Epidemiology, and End Results database was examined to identify cases of female breast cancer diagnosed between 2000 and 2007. The optimal cutoff value for young age was determined using the X-tile program (Yale University, version 3.6.1). Age-specific gene expression profiles were explored using RNA sequence data from the Cancer Genome Atlas database.The age of 40 years was determined as the optimal cutoff value. Among 94,087 patients, 12,755 were aged 40 years or younger (younger group), and 81,332 were older (older group). The 5- and 10-year cancer-specific survival rates in younger and older groups were 88.74% and 80.65%, respectively, and 93.22% and 88.43%, respectively (P < .001). Univariate and multivariate analyses indicated younger patients had worse prognosis. Subgroup analysis according to estrogen receptor (ER) showed the risk for cancer-specific death of ER-positive (ER+) younger patients increased by approximately 2 times (hazard ratio, 1.96) compared with ER+ older patients. We failed to find any age-related gene in 509 patients after adjusting according to subtype (50-gene prediction analysis of a microarray) and histological type.The age of 40 years is a reasonable cutoff value for defining “young.” Young patients with breast cancer, especially those in the ER+ subgroup, have worse prognosis. However, we found that young breast cancer is not a unique biological entity, and therefore, a lack of new potential targets.