In my experience it is rare to see a Rosh Yeshiva or other prominent Rabbi declared to be senile and no longer working at full mental capacity. Recently such a proclamation was made about one Rosh Yeshiva however this was due to a legal situation. I think it is fair to assume that there are other Gedolim or older Roshei Yeshiva who are in a similar situation. I understand these things are not made public out of respect, but halachically I would guess that a person who no longer has all his faculties cannot be relied upon for psak. At what point can/should an assumption be made by the general public that a certain Rav is senile and can no longer be relied upon, even absent a medical diagnosis?