The purpose of this study is to determine the best dose of an immune suppressive drug, cyclophosphamide, following standard stem cell transplant in older patients (at least 65 years old). Participants will undergo a stem cell transplant with cells from a related donor and will receive a standard or reduced dose of cyclophosphamide that is routinely administered after a transplant procedure. They will also undergo heart MRI scans and complete a series of assessments of physical and cognitive function. The study will perform the transplant process as would be done routinely, except a reduced dose of cyclophosphamide will be given to some transplant patients and standard doses to other patients in order to determine whether lower doses can be safe and effective. Researchers also want to learn what happens when young stem cells are transplanted into older bone marrow recipients. Markers of aging will be studied on the stem cells and bone marrow before and after the transplant procedure. The U.S. Food and Drug Administration (FDA) approved cyclophosphamide for treating various conditions, but its use in transplant, while standard for many years, has not been approved.