Age and physiologic status, such as menopause, are risk factors for breast cancer. Less clear is what factors influence the diversity of breast cancer. In this study, we investigated the effect of host age on the distribution of tumor subtypes in mouse mammary chimera consisting of wild-type hosts and Trp53 nullizygous epithelium, which undergoes a high rate of neoplastic transformation. Wild-type mammary glands cleared of endogenous epithelium at 3 weeks of age were subsequently transplanted during puberty (5 weeks) or at maturation (10 weeks) with syngeneic Trp53-null mammary tissue fragments and monitored for one year. Tumors arose sooner from adult hosts (AH) compared with juvenile hosts (JH). However, compared with AH tumors, JH tumors grew several times faster, were more perfused, exhibited a two-fold higher mitotic index, and were more highly positive for insulin-like growth factor receptor phosphorylation. Most tumors in each setting were estrogen receptor (ER)-positive (80% JH vs. 70% AH), but JH tumors were significantly more ER-immunoreactive (P = 0.0001) than AH tumors. A differential expression signature (JvA) of juvenile versus adult tumors revealed a luminal transcriptional program. Centroids of the human homologs of JvA genes showed that JH tumors were more like luminal A tumors and AH tumors were more like luminal B tumors. Hierarchical clustering with the JvA human ortholog gene list segregated luminal A and luminal B breast cancers across datasets. Lastly, these data support the notion that age-associated host physiology greatly influences the intrinsic subtype of breast cancer.