In fiction, there are two kinds of character: dynamic and static. Dynamic characters experience growth and change. At the end of the story, dynamic characters have had many experiences that have caused them to change and develop from the individuals they were at the beginning of the story. Main characters and major characters are typically dynamic. Static characters, on the other hand, experience little or no growth or change during the course of the story: they are the same at the end of the story as they are in the beginning. These are typically minor characters in stories, or stock characters who serve as plot devices rather than fully developed character.
Little Dragon Girl was the second main character of ROCH. Indeed, judging by the title of the book, she shares equal billing with Yeung Gor (and a certain Divine Condor).
Whereas Yeung Gor was definitely a dynamic character who experienced growth and change, Little Dragon Girl didn't really change much at all...and I don't mean just her appearance. In terms of personality and worldview, how different was she as of the final page of ROCH from how she was depicted in her first appearance? Hardly any at all. By not aging her and not altering her personality, Jin Yong was probably trying to emphasize her consistency, but that consistency can also be read as indicative of a static character, which is all right for minor characters but a huge flaw for a MAIN character.