Longitudinal Assessment of Brain Structure and Function in Juvenile-onset Huntington's Disease

The goal of this observational study is to learn about brain development in Juvenile-onset Huntington's Disease (JoHD). The main questions it aims to answer are: * Is brain development different in JoHD than Adult-onset Huntington's Disease (AoHD)? * Can reliable biomarkers for JoHD be found in brain structure and function? Participants will be asked to complete cognitive tests, behavioral assessments, physical and neurologic evaluation, and MRI. Data collected will be compared to populations who are at-risk for HD and who have been diagnosed with HD as adults.