I've wondered this myself. My grandmother expected the end to come when she became a Witness in the '40s, pregnant with my mother. When she was in her 90s and saw her health declining before the world ended, I wonder if she had any doubts. The thing is, once someone is past a certain age and their brain has become implastic, they don't seem capable of introspection and belief change, therefore I'm not sure they really are capable of doubts either. Armageddon is always just around the corner for them.