I know plenty of IRL people in their 20's that are "mentally" children.

It should be important to note that as a character, Raphtalia's childhood came to an end when she and her friend's were captured by slavers after her town was destroyed in the first wave and had to endure the torments that followed. As she is now, she's far more adult than a child as her experiences have matured her emotionally as well as physically.

Well, with at least one exception… somebody probably needs to have "the talk" with Raphtalia, and soon