Additional Study Information: The purpose of this research study is to better understand the brain's dopamine system among older adults with depression. In this research study, you will receive a medication called carbidopa/levodopa (Sinemet). This medication is approved by the Food and Drug Administration for the treatment of Parkinson's Disease. We are using carbidopa/levodopa off-label in this study to see whether it is capable of improving depressive symptoms as well as mental and physical slowing that occur as people age. In addition, you will be required to complete certain exercise routines, in order to determine whether treatment with physical activity affects depression and slowing.