Such people exist, and for that matter some women assume such a persona for sexual play reasons.
Not a big fan of this type of thing myself, but I think it is stretching the term “pedophilia” well beyond any reasonable meaning when you apply it to a 21 year old girl simply because she is somewhat immature and naive.
Pedophilia refers to sexual desire for prepubescent children. This doesn’t sound much like that to me. It sounds more like the lead female character is mentally and emotionally a young teenager but fully mature sexually. You can call that a number of things, none of them particularly complimentary, but I don’t think it’s fair to call it pedophilia.
Do I think the storyline is somewhat repulsive? You bet.
Do I find it REALLY odd that it is so popular among young and middle-aged purportedly feminist women? Absolutely. They obviously identify with the young woman “heroine,” which says something really odd about what they find erotic or entertaining.
Actually they don't. They identify with the good girl changes bad boy to only semi-bad boy who's true and faithful to only her meme that has existed for centuries. Cut through everything else, and that's this story's hook: Good girl reforms bad boy, film at 11.