It's written by people who have no realistic clue what normal behavior for any age is. It's not meant to be realistic, so it's dumb that it bothers me, but there it is. I can get behind age play where it's general kink play with spanking and whatever because that's not a specific age and not meant to be anything realistic. But a lot of age regression fic I've read is very focused on "what age are they" (like figuring out what 'type' of Little they are is a big plot point) and "proper" childcare techniques by the "caregivers" and it's just all off and clearly written by people who have no clue.