First--
Yes, I do like Lord of the Flies...not my favorite book, but it's good...
And I would count it as an "adult" novel, the same way I'd count SFR and THG--
I'd count most things that aren't, say, "Charlie and the Chocolate Factory" or "The Phantom Tollbooth" and such "adult" novels, though, really...
I'd prefer to just drop the age pretense and call them novels period, full-stop, as, well, some kids read faster than others, some kids will be reading Austen and Twain in middle/early high school...some will be reading Lord of the Flies...some will be reading The Hunger Games...and so on.
The age distinction, at a certain point, becomes silly.
Again--once the book has an adult message, it ceases to be merely a kid's book and becomes a novel; more adults, true, may read Conrad than Collins, and more teens may prefer the exploits of Katniss'-crazy-knife-girl-assailant (oddest part of the whole book...suddenly, some random girl decides to have a random vendetta and give a sadistic villain speech and get ready to go Ledger Joker on Katniss' face...WTF, why doesn't THIS kid have more of a focus, forget the trite love interest boy, I want to know how this girl got so fucked up so fast!) to those of Kurtz...
But regardless, they're still NOVELS.
I reject the whole young adult/adult dichotomy, people read different material at different levels at different times.
As odd as it is for me, an elitist English major and self-professed fan of the Bard to say...
Shakespeare isn't necessarily always or entirely "more adult" than Suzanne Collins--that's for the author to decide.
Now, is Shakespeare more complex?
By leaps and bounds he is.
Is Shakespeare the more mature author/has the more mature audience on average?
A bit open to interpretation, and the Bard could be pretty silly...but probably so.
Is Shakespeare the more influential author?
Incredibly so (in fairness, Collins' books are new, but still...500 years hence, RIII > THG)
Is Shakespeare the better author?
That's a matter of taste...but if we're going to go by technical prowess and popularity and complexity and influence and all of that...you'd need a lawyer who could get Charles Manson out of jail to make that argument stick, I think (or, take the shorter way--become a successful writing legend in your own right, like Leo Tolstoy, and then you can follow his footsteps and call Shakespeare garbage all you like by standing on a platform built by such mountainous works as "War and Peace" and "Anna Karrenina.")
;)
But all that aside, there's nothing necessarily more "adult" about Shakespeare...
He and Suzanne Collins are in the same arena--the fact that he and many, many, MANY authors would go on the unofficial ranking lists, so to speak, ahead of Collins, who probably couldn't right now crack a Top 200 (not necessarily a slight to her, again, I think she's decent if lazy and flawed, that's more a statement of just how many great authors have existed so far in history) doesn't mean Suzanne Collins is out of the "adult" literature (not that kind!) arena.
She doesn't get her own subset where she can be queen; sure, we may group her in with other authors who had similar target ideas and formats and so on, we may group her with the dystopians with Orwell and Huxley, or with her modern contemporaries...the same way we'd group Shakespeare with Marlowe and Kyd, historically, or, in terms of influence, have him right up there with Dante and Homer and their ilk...
But, at the end of the day, we can compare anyone to anyone, regardless of subset--
We can set Shakespeare up against Oscar Wilde or Sophocles or Albert Camus and so on.
But that doesn't just go for the big-name writers you can somehow turn into a semi-meme on an Internet forum by mentioning them so damn much. ;)
If we want to compare Orwell to Collins, or Collins to Golding, or Conrad to Collins and Orwell, or any combination of those, and others...we can--they're writing about adult ideas and have adult messages, ergo, they're fair game for serious, adult criticism.