Elders are supposed to take extenuating circumstances into account, and dementia would most definitely fit that.
I recall a DO (yes many years ago) explaining the "rule of thumb" was that if the person was capable of living independently, they would be "accountable" for DFing - can he hold a job? Care for his own personal hygiene? etc. If so, then he can face a judicial committee.
So regarding dementia, it would largely depend where on the spectrum of dementia the person fell. If mild, the person might well be considered "accountable". If farther along, probably not.
That said, the "quality" of elders varies wildly. Some are incredibly proud, or egotistical, or stupid, or on a power trip, or "all of the above". It's an absolute crapshoot which 3 elders end up on a committee - often it's whichever 3 have openings in their schedule for a particular evening or the 3 who most recently haven't "handled" any judicial cases.
So, it would not surprise me one scintilla if some, or many, judicial committees ended up DFing folks with more severe dementia, mental illness, brain damage, you name it.