Brain shrinkage is normal as we age into our 70's, 80's and beyond. Moreover, upon autopsy, many brains show damage consistent with Alzheimer's. One of the other frequent challenges of age is the decline of one's sense of balance -- something we REALLY need for safe flying.
But the brain is a tricky organ. Like several other of our organs, it seems to have considerable excess capacity that can be activated after limited damage to the primary portions. Many of those whose brains have Alzheimer's-like damage in fact showed no symptoms during their lives.
Good nutrition, exercise and continued stimulation and challenge seem to stave off some brain-ageing symptoms.
So, yes, each elderly pilot is a unique case. Some of 'em are way sharp; others are scary.