Into the Fog: Living with Early-Onset Alzheimer's | WebMD

Early-onset Alzheimer disease (AD) is defined as 
having an age of onset younger than 65 years. 

Early-onset AD comprises about 5% to 6% of cases of AD
and includes a substantial percentage of phenotypic variants that differ from the usual amnestic presentation of typical AD. Characteristics of early-onset AD in comparison to late-onset AD include a larger genetic predisposition (familial mutations and summed polygenic risk), more aggressive course, more frequent delay in diagnosis, higher prevalence of traumatic brain injury, less memory impairment and greater involvement of other cognitive domains on presentation, and greater psychosocial 
difficulties.