Breast cancer is the most commonly diagnosed cancer in women and a leading cause of cancer death in women. Studies that examine the effects of certain environmental factors, which can mimic estrogen function and thus mediate breast cancer development, are crucial to broadening understanding of molecular cancer mechanisms. Subsequent translation from in vitro to in vivo studies and ultimately to possible clinical therapies requires the development of effective techniques to study mechanistic function at each stage. Here, we present data describing the use of zebrafish xenotransplantation as an in vivo model for studying breast cancer progression and metastasis. A younger (~0.5 year, median 2.9cm length) and an older (~1.5 years, median 4.1cm length) group of zebrafish were immunosuppressed by dexamethasone treatment and transplanted with MCF-7/GFP breast cancer cells. By fluorescent imagery, tumor area and metastasis in both groups were observed over 7 weeks to compare transplant efficacy. In addition, a systematic method, using python codes, was developed to analyze images. Throughout the study, the younger group showed significantly lower transplant efficacy, with most fish showing no signs of tumor presence by 2 weeks post injection. The older group showed high transplant efficacy, with all individuals within the group showing tumor present throughout the length of the trial period. We conclude that zebrafish age is a determining factor in transplant efficacy. We speculate that the variable immune robustness of younger versus older fish impacts dexamethasone immunosuppression. In turn, this effect determines xenotransplant efficacy.