In a study of breast cancer, a mother was considered exposed if she first gave birth at 30 years of age or older. Among 1,628 mothers who were exposed

In a study of breast cancer, a mother was considered exposed if she first gave birth at 30 years of age or older. Among 1,628 mothers who were exposed, 31 subsequently developed breast cancer. Among the 4,450 mothers who first gave birth before the age of 30, 65 subsequently developed breast cancer. What is the appropriate conclusion regarding the association between age at first birth and breast cancer?