To treat this question seriously for a minute:
From my non-expert POV Stein is not exhibiting signs of dementia. He has allowed deeply held biases to control his reason on some topics, but that is not remotely a symptom of dementia. He is still articulate. He still can make the case for his POV reasonably well.
I originally thought this question applied to McCain. I think there are much stronger signs of at least reduced mental functions that are probably age related in McCain than in Stein. And it seems within the realm of possibility that some of those signs might be very early signs of mental problems down the line. McCain's verbal gaffes are not that far out of the norm, but some of them like the notion that Iran might be harboring Al-Qaida seem to be so bizarrely off that I have wondered if McCain isn't dealing with some mental acuity reductions. Probably though, the idea that any of this is early signs of dementia in McCain stretches the evidence too far.