Amyloid plaques in postmortem human cortex correlated with the proportion of microglia that were activated, not with microglial numbers. Tau pathology and cognitive decline come later.
Frail people may be more likely to have Aβ plaques and neurofibrillary tangles; when they do, they are more likely to have dementia. Physical activity correlated with better global cognition, regardless of brain pathology.