What is "Young Adult?" (In which the writer attempts to engage in academic inquiry and concludes with opinionated and unresearched ranting)

From what I understand, classifying literature as "young adult" is relatively new.  Along with this comes new research, definitions, experts, and more questions than answers.

For example: Who qualifies as a young adult, and why?  Our standards for adulthood are strange: as Joseph Campbell noted so significantly in Power of Myth, we have no one cultural definition for adulthood except age.  At 16, you can get a permit; at 17, you can see R rated movies by yourself; at 18, you're legally an adult; at 21, you can drink.  But at what age does a human being know sex, poverty, violence, injustice?  At what age does a human understand life?  At what point does a child become an adult, and what is the meaning of the time in between?  As far as I can tell, understanding is a life-long process, most adults I know lack maturity in some way or another, and many children know of more than we adults are willing to admit--some children know more than we adults know.

Is YA lit written for, by, and/or about young adults?  What of all these writers--adult writers--who suddenly decide to flood the YA market with their work, using the formula less grammar + lower age of characters + popular culture references = young adult?

Most importantly, who is allowed to read young adult literature?  Maybe the definition of a young adult lies here: those with the desire to grasp for more than has been assigned or designated to them.  Then the question becomes one of availability: are we allowing children to grow into adults at their pace, or are we stunting their growth by fixing their potential to numbers?

Right now, we have recommended summer book lists separated by grade, but does a spectrum exist that factors in a person's maturity, interests, and reading level to find books that will foster growth?  For example, if an eighth-grade student reads at a second-grade level, they shouldn't have to read books that have the maturity of a second-grader. Likewise, if a fourth-grader has excellent reading skills and devours a Beverly Cleary book in an hour, but isn't interested in the horny thoughts of older kids, do modern books exist for them or are they limited to Victorian classics?  Summer book lists are organized by where a child's reading level SHOULD be.  That's fine for all the fourth graders taking the eighth-grade book lists and feeling cool, but probably not as awesome for the fourth graders who wonder why they're stuck reading picture books filled with chubby, rosy toddlers.

Of course, all of this ties in with further rants about how education systems pass or are forced to pass kids en masse grade after grade without knowing necessary material, how--again--our notion of adulthood is based on age rather than achievement, and how we forget that every student--youth, child, and adult--is an individual.  But what do I know of such things?