It’s not news to anyone that adults are reading books marketed to young adults. Harry Potter and Twilight may have made this phenomenon more socially acceptable, but they hardly started it. Think Tolkien and Mark Twain, back before the publishing industry had articulated marketing categories such as “middle grade” and “young adult.”
What’s new is that publishers are increasingly adjusting their marketing strategies to work with this reality instead of against it.
More and more titles are being published as YA in the US and as adult in other countries, and sometimes vice versa. Margo Lanagan’s Tender Morsels, originally published as an adult title by A&I in Australia, was published as YA by Knopf in the US. The Book Thief was published as an adult title in Australia.
And publishers in the UK regularly publish YA and adult editions simultaneously. This is certainly the case for His Dark Materials.
From my own experience writing middle grade and YA fiction (and working very closely with other writers) is that one of the most difficult questions you can ask an author is the age target of the book. That question confounds me like no other.
When I was writing Kaimira, I did have a certain audience in mind. They were people like me, people who love losing themselves in massive fictional worlds. They are readers above all else, but they enjoy immersive storyverses in any format, from fan fiction to gaming.
But this audience I had in mind did not have a specific age. They could just as easily be 30 as 13.