Alzheimer's disease (AD) is usually associated with aging, age being the principal identified risk factor. However, younger subjects also develop AD and the prevalence of early onset AD is unknown. It is estimated that about 30 000 subjects develop symptoms of AD before the age of 65 in France. There is evidence that early onset AD differs from AD in older patients. In particular, clinical and neuroimaging studies suggest early involvement of neocortical brain regions and their functions in early onset AD, while mediotemporal areas and memory might be more involved in late onset AD. These differences could partly explain the atypical clinical and imaging features of younger patients, the diagnostic difficulties in these patients and the specific problems related to medical care of this age group. The present study uses a multidisciplinary approach with longitudinal followup in order to establish the impact of age on the clinical and neuroimaging picture of sporadic AD in a multicentric setting. Another aim of the project is to describe for each age group, and in particular for the younger patient group, the functional impact of disability in everyday life on both, patients and caregivers.