Agreement with Ruth Graham at Slate on reading Young Adult novels as a non-young adult, with one exception:
I don't begrudge anyone reading anything. I spend time on the internet and with light reading, as well. Literary fiction is certainly in thrall to a handful of techniques, styles, and topics that may be of limited interest, or can become of limited interest if you've read enough examples of them. Tastes change over time, people can/will run out of preferred works by preferred authors, and there's great virtue in changing up the types of authors or books one is reading.
When I think about the really pleasurable experiences of reading in my life, they are all in the tackling of something that seemed far outside by ability or interest but proved itself to be worth the work invested. "Pleasure" is the right word: related to but distinct from the pleasure that comes from reading in general. Part of the thrill of working through my current novel--The Mill on the Floss--is the fact that I began it without much success on several occasions before finding myself interested and able; the initial difficulty and the work are the pleasure of it, no less than the story and its telling; those are the parts of the experience of reading that are mine.
If the debate between the two camps seems stultifying, the primary cause lies in how the YA-ers and the lit fiction-ers treat the two as mutually exclusive options. That literary fiction has mostly surrendered writing about or considering the feelings of childhood and adolescence is a great loss, and better that YA pick it up than no one at all. But before there was a divide, the world had no shortage of literature that spoke to and was usefully adopted by the young.
I am old enough to remember, if slightly, the world of fiction young adults were supposed to inhabit before there were books specifically written for them: David Copperfield, Huckleberry Finn, Little Women, Jack London, Jane Austen or the Brontes, Kerouac or J.D. Salinger, and many others. That is to say: young adults read adult fiction written for adult audiences, but whose subjects were people of their own approximate age and experience in life. Thus you get, ideally, not only the accurate representation of how it feels to be at a certain age, but the knowledge that one will one day surpass all those feelings and come to see the world differently, with more perspective and with greater equanimity. The Mill on the Floss could hardly be matched as an account of a certain part of childhood, where intentions are incompletely transferred to action and emotions come and depart with startling intensity (Maggie pushing her cousin into the mud gets the jumble exactly right). It will not be consumed with immediacy, because its world is different and requires translation, but adult life is about encountering people who have had different experiences and learning how to translate and understand them.