The reasons for the myth's persistence continue to fascinate me. Partly it's because people are such poor statisticians. We remember the two or three cases of people we know who prove the myth to be true and disregard the dozens, if not hundreds, of others in our acquaintance who continue steadfastly through the middle decades of life. There's also a fascination and allure to the midlife crisis.
There is a widespread public perception that dementia can lead to a loss of a sense of self, but this notion has not been rigorously investigated. One way to study this is to look at actual cases of brain degeneration, and see if the damage is linked to identity changes perceived by others. Do people with specific kinds of brain damage become no longer themselves?