When you ask "Why ?" it would be nice to first make sure that the XI statement is true. Different people can see themselves in a mirror in different ways. Thin fat, old - young, white - black, people - hobbits. This is a relative and subjective perception of some people, and not an objective truth that can be recognized as "true" or "untrue".
And don't you really understand why? "Why do old people feel like they are trapped in a decrepit unfamiliar body?" Some people feel that way, yes. Do you really need lengthy explanations from specialists why being decrepit is unpleasant and you don't want to?
I dare to suggest that old age pushes to remember the past youth due to the decline of life. And the image of an 18-year-old boy is a projection of the best period of his life