What is Appropriate for Young Adult (YA) Literature?

Emily Witt related in a New York magazine “Intelligencer” article that publishers have started referring to Young Adult (YA) literature as New Adult (NA) literature. Per the American Library Association (ALA), a young adult is someone between the ages of 12 and 18-years-old. But is content that is “appropriate” for 18-year-olds “appropriate” for 12-year-olds?

Let’s look at Lauren Myracle’s ttyl. Here’s Amazon’s plot summary of the YA\NA book:

This funny, smart novel follows the friendship of three 16 year old girls [Angela Silver Madigan, "Maddie" Kinnick , and Zoe Barrettas] they experience some of the typical pitfalls of adolescence: boys, queen-bee types, a flirty teacher, beer, crazy parents, and more. Lauren Myracle has a gift for dialogue and characterization, and the girls emerge as three distinctive and likable personalities through their Internet correspondence. This light, fast-paced read is told Entirely in instant message format, the first book ever for young adults to be written so.

Here are some content that I found surprising and wrote about in this “runaway” best-selling book written for teens:

Margaret, Angela’s high school friend, knows how to squirt. 

Margaret desperately wants to have a relationship with a bohemian Georgia State University (GSU) student, but she didn’t realize that 15-year-old Angela had a crush on the collegian as well.

It excited Angela to fantasize about her classmate's "summer sausage" while she did her homework.

15-year-old Maddie danced topless on a table at a GSU frat party as camera phones snapped away.

And in terms of the teacher/student relationship, Zoe was initially attracted to Mr. H, her English teacher, because, "... he's NICE. cuz he treated me like i was a person instead of a kid. that what was so great - we were just ppl having a discussion." Zoe and her teacher went out several times, but when Mr. H invited Zoe to join him in the hot tube wearing a pair of Speedos, he "paralyzed" her and she ended the relationship.

Apparently, some parents didn’t think that ttyl was “appropriate” for 12-year-olds, because the book is on the American Library Association's list of books parents want banned.

But what is appropriate for YA\NA “literature”?