[Plough_Jogger]

There is also the possibility that the causation happens in reverse: those with early symptoms of dementia could be self-medicating with excess caffeine.

[maxerickson]

Or the coffee could be preventing other health problems so that more drinkers make it to dementia.

[wutbrodo]

I doubt the study would fail to control for this; it's literally the most basic thing you can control for.