By the time the actors I mentioned were written about as having dementia, many film fans had already assumed they were dealing with dementia precisely because they were no longer being written about or seen in the media much. Such speculation about Jack Nicholson, for example, was rife on film forums well before those paparazzi images appeared.

Or possibly, actors who still have their faculties tend to keep acting, even into advanced age. Not sure if that's true, but even the perception of that being true could lead to these kinds of assumptions.