The ultimate goal of this project is to develop treatments for the prevention of AD in this population. Because if the investigators would know at what age the slope of transition to dementia is steepest, a study could be done in a specific age range and require much fewer subjects and time and expense than a study in the general population (where it is currently impossible to determine which healthy individuals will likely develop dementia over the next few years and even if there were, thousands would have to be screened to ID these few). A study in the general population might need to enroll tens of thousands of subjects to observe sufficient transitions to dementia. The investigators estimate that such a study in DS would require about 170 subjects per arm for a 36 month study. Success in such a study would provide the confidence to commit the resources for a larger study in the general population.