early onset dementia early onset dementia early onset dementia is dementia in which symptoms first appear before the age of 65. the term favored until about 2000 was presenile dementia; young onset dementia is also used. early onset dementia may be caused by degenerative or vascular disease, or it may be due to other causes, such as alcohol-related dementia and other inflammatory or infectious processes. early-onset alzheimer's disease, vascular dementia and frontotemporal lobar degeneration are the most common forms of early onset dementia, with alzheimer's accounting for between 30 and 40%. early onset dementia may also occur, less frequently, in the lewy body dementias (dementia with lewy bodies and parkinson's disease dementia), multiple sclerosis, huntington's disease and other conditions.