But increasingly, adults are reading YA books with no ulterior motives. Attracted by well-written, fast-paced and engaging stories that span the gamut of genres and subjects, such readers have mainstreamed a niche long derided as just for kids.
Thanks to huge crossover hits like Stephenie Meyer's bloodsucking "Twilight" saga, Suzanne Collins' fight-to-the-death "The Hunger Games" trilogy, Rick Riordan's "The Lightning Thief" and Markus Zusak's Nazi-era "The Book Thief," YA is one of the few bright spots in an otherwise bleak publishing market. Where adult hardcover sales were down 17.8% for the first half of 2009 versus the same period in 2008, children's/young adult hardcovers were up 30.7%.
"Even as the recession has dipped publishing in general, young adult has held strong," said David Levithan, editorial director and vice president of Scholastic, publisher of "The Hunger Games," as well as of J.K. Rowling's Harry Potter books, the series largely credited with jump-starting this juggernaut of a trend.
"You go on the subway and see 40-year-old stockbrokers reading 'Twilight,' " said Levithan, himself a YA author. "That wouldn't have happened five years ago."
Levithan added that passing "the mother test" is an indication that a title could go wide. "If a lot of us on staff are sending a book to our mothers because it's really engaging literature, that's a good sign."
Books that have passed the Scholastic mother test? Judy Blundell's "What I Saw and How I Lied," which won a 2008 National Book Award, and the wolf love story "Shiver" by Maggie Stiefvater.
According to Kris Vreeland, children's department manager for Vroman's Bookstore in Pasadena, "You have a lot of different people coming to young adult in a lot of different ways."
Often, word of mouth will bring a teen title to an adult's attention, Vreeland said. Such was the case with the "Twilight" series, which has sold more than 85 million copies worldwide since the first book was published in 2005.
Other times, it's an award. When Sherman Alexie's young adult debut, "The Absolutely True Diary of a Part-Time Indian," won the National Book Award in 2007, it lent credibility to the entire genre.
"One strong writer leads to exploring that area more, so you've got several now who are leading people into all kinds of directions," Vreeland noted. "You can go the whole gamut: sci-fi, fantasy, mystery, historical fiction, romance, realistic fiction, humor. There's a lot of good stuff going on."
Add the growing number of movies made from kids' books, such as "The Sisterhood of the Traveling Pants" and "Percy Jackson & the Olympians: The Lightning Thief," as well as all the successful adult authors -- James Patterson, Carl Hiaasen, Francine Prose and Terry Pratchett -- now writing for younger readers, and you've got a phenomenon "that extends beyond the gatekeepers who want to know what their kids are getting into," Vreeland said.
Christel Joy Johnson is one. The 36-year-old actress doesn't have children, but she's an avid reader of young adult science fiction.
"There's something really wonderful about taking the journey with someone of that age. One of the main reasons I'm attracted to YA literature is just the openness of the characters," said Johnson, who recently finished the necromancer tale "Sabriel," by Garth Nix.
"I think part of the reason we're seeing adults reading YA is that often there's no bones made about the fact that a YA book is explicitly intended to entertain," said Lizzie Skurnick, 36, author of "Shelf Discovery," a collection of essays about young adult literature from the 1960s and 1970s.
"YA authors are able to take themselves less seriously. They're able to have a little more fun, and they're less confined by this idea of themselves as Very Important Artists. That paradoxically leads them to create far better work than people who are trying to win awards."
According to Skurnick, who also reviews adult fiction for publications including The Times, YA books are "more vibrant" than many adult titles, "with better plots, better characterizations, a more complete creation of a world."
And often, those worlds are steeped in the imagination. In Patterson's "Maximum Ride" series, a 14-year-old girl leads a pack of laboratory-bred, winged teens on various adventures. In "I Am the Messenger," Zusak writes about a teenage cab driver who becomes an inadvertent hero, prompted by anonymous messages that lead him to specific addresses at specific times.
Many of today's young adult authors were born and raised in the 1960s and 1970s, when YA began to move beyond the staid, emotionless tales of Nancy Drew and the Hardy Boys in favor of more adventurous work from Judy Blume, Madeleine L'Engle and Robert Cormier. Now, they're turning out their own modern masterpieces.
"There's some amazing, vibrant, fantastic literature in the YA venue," said Cecil Castellucci, a young adult author who recently started the Pardon My Youth book club at Skylight Books in Los Feliz to "help people understand that YA literature is not just for young adults."
According to Castellucci, author of "Boy Proof," "Beige" and other titles about misfit teenage girls, we're living in the golden age of young adult literature.
"As a YA author, I get tired of being asked, 'When are you going to write a real book?' " she said. "As if a YA book is not a real book."
Ask any of the genre's growing legion of fans: It is.