I experienced one of those sitting next to me on an RJ, window seat in the exit row. The guy had trouble walking (was helped onto the aircraft by a traveling companion who appeared to be his middle aged daughter) and finding his seat, didn't understand and needed help to fasten his seatbelt, couldn't feed himself snacks very well, etc., so he seemed to be senile as well as disabled. The flight was a bit late and the two seats that were assigned (windows in the same exit row, where I had the aisle seat next to the guy and someone else was seated next to the daughter) at the gate seemed to be the last two available seats on the flight. Still, the GA and FA should have noticed that both of them (she would have needed to help him in an emergency) upon boarding and again when both were required to get a response to the standard exit row questions. I tried to say something to the FA but feared being booted from the flight myself, so I was reluctant to say something like "Just look at him: he's obviously both too incapacitated to open the door and too senile to be understand what to do. Are you kidding that you think it's OK to place him in an exit row?" Instead, I told myself that since he was frail, I could just shove him out of the way in an emergency and open the window exit myself.