I don't know why I'm playing White Knight for Martin but: I have read interviews where he justifies his use of sexualized 13 year olds as based in actual medieval practice. They didn't have the concept of adolescence, you went directly from childhood to adulthood.
I also think that since the chapters are written from the POV of the characters, the level of description fits. In other words, the quote you produced, while icky realizing that it is about a 13 year old, has both a historical precedent and the chapter is from that character's POV and thus that would be a pretty significant detail she would be noticing.
I'm afraid I'm being too knee jerk based on how some of the reviews of the show tend to take aim at fantasy as a genre rather than the actual work presented. I'm not a fantasy reader, this series and Lord of the Rings is about as deep as I go, but to have The New York Times get so condescending and describe the work as "boy fiction" really rubs me the wrong way.