Is aging part of Alzheimer's disease, or is Alzheimer's disease part of aging?

For 70 years after Alois Alzheimer described a disorder of tangle-and-plaque dementia, Alzheimer's disease was a condition of the relatively young. Definitions of Alzheimer's disease (AD) have, however, changed over the past 30 years and under the revised view AD has truly become an age-related disease. Most now diagnosed with AD are elderly and would not have been diagnosed with AD as originally conceived. Accordingly, younger patients that qualify for a diagnosis of AD under both original and current Alzheimer's disease constructs now represent an exceptionally small percentage of the diagnosed population. The question of whether pathogenesis of the "early" and "late" onset cases is similar enough to qualify as a single disease was previously raised although not conclusively settled. Interestingly, debate on this issue has not kept pace with advancing knowledge about the molecular, biochemical and clinical underpinnings of tangle-and-plaque dementias. Since the question of whether both forms of AD share a common pathogenesis could profoundly impact diagnostic and treatment development efforts, it seems worthwhile to revisit this debate.