YouTube user Pop Culture Detective has created a fairly comprehensive, insightful essay about the film trope “born sexy yesterday,” in which grown (and frequently sexualized) women have the minds of children. From the essay:
“Born Sexy Yesterday” is about an unbalanced relationship. But it’s also very much connected to masculinity. The subtext of the trope is rooted in a deep-seated insecurity about sex and sexuality. The crux of the trope is a fixation on male superiority. A fixation with holding power over an innocent girl. But in order to make that socially acceptable, science fiction is employed to put the mind of that girl into a sexualized adult woman’s body.
It’s a fantasy based on fear — fear of women who are equal in sexual experience and romantic history, and fear of losing the intellectual upper hand to women.
Seeing all the examples laid out like this makes clear how ubiquitous and pernicious this trope really is.