In my view, there are a lot of people who really don't know what they are doing when they vote. The difference is, there is no one in the voting booth moving their hands, or telling them what to do.
As others have said, if we allowed people to vote strictly on intellectual ability, and knowledge of what is going on, many, many people would be disenfranchised. And that would not be a good thing. My concern is where there are people who have someone else "pull the strings" for them, literally, where the voter him/herself has no real choice in the matter, or even knowledge of what is happening.
I agree, if a person has no idea whatever of what is going on and another person is just using that person as another vote then that is wrong.
It is just that I don't think the political arena (so to speak) is the place to diagnose degrees of dementia nor should it ever be in a position where it recommends people get tested for dementia.
I personally think any good that can come of it is outweighed by the bad that can come of it.
Take the florida election in 2000 as an example. They really screwed up then and a lot of people didn't get to vote because they were "mistakenly" put down as felons and so were disqualified to vote. I sure wouldn't trust this same state with the same group of people whoever they were (democrat or republicans) to decide who has dementia and who does not.