We found that compared with the overall and older breast cancer population, a lower proportion of young adult patients had stage IV cancer, lobular carcinoma, HR−/HER2+ expression, only surgery treatment, and BMI ≥ 24 kg/m2, whereas a higher proportion had surgery combined with adjuvant and neoadjuvant therapy, normal and underweight BMI, current cigarette smoking status, and current casual alcohol drinking status. This evidence concerns the gene ERBB2 and cancer.