Unfortunately its not. I know how we want to be all politically correct in the western world and its true that anecdotal evidence suggests that people can be active at 85+ years. Every one should be thus evaluated individually. However, specially in North America, the older we get, our life styles are so sedate and our food is so rich with useless sugar calories that our bellies go the shape of a large zero and our body goes the shape of a tear drop and our mobility decreases. We also tend to gain more weight. We generally get out of the habit of learning and thus our brains take much longer to learn new skills like flying. This does not obviously apply to everyone because some people have better health, better fitness, food and exercise but it stands true in general. PC stuff not withstanding. Its the bitter truth we may not want to face because it feels bad. That has been my experience and my opinion anyway.
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.