Our analysis of gene expression changes in the brains of Alzheimer's patients confirms that AD is both similar and distinct from the process of normal aging. Although each brain was captured only in a particular (postmortem) state and was not studied longitudinally, we can assemble these data as a function of time to propose a few generalized aging trajectories. BioAge and chronological age showed a significant association in non-demented individuals and no association in AD patients, who had consistently high BioAge scores regardless of their chronological age. We attribute this observation to a difference in the strength of the aging drivers, distribution of the aging rates, and different causes of death in the two cohorts. In non-demented individuals, the drivers of aging were weak; the rates of aging were relatively slow and consistent across the population; and, in the absence of unnatural causes, death was likely related to aging issues other than the health of the brain. Since non-demented individuals likely died from causes largely unrelated to neurodegeneration, each individual death is conceptually a random event along the generalized brain
