One exception that really stood out for me was Morgan Keyes's Darkbeast. It's a fantasy story about a twelve year-old girl named Keara. On her world, every child is given a "darkbeast" (an animal familiar) who accompanies them everywhere. The darkbeast's job is to provide a companion and listening board for the child. More than a pet, the children develop a telepathic nurturing bond with their animal. However, when they turn twelve, they are forced as a rite of passage to kill their companion (and basically cast aside childish things). Keara doesn't want to do this and learns of an underground of older people (called "darkers") who secretly have kept their darkbeast alive, flouting societal convention and refusing to break from their childish behavior.
There's a lot going on in the book (and its sequel Darkbeast Rebellion) but the message that really resonated with me was the darkers's resemblance to the ageplay community -- its need to stay hidden because of society's hatred and its own nurturing within. Keyes also does a wonderful job exploring how some of the straights (who actually did kill their darkbeasts) regret their decision but manifest their regret through jealous hatred for the darkers living the life that they cannot.
I think this Darkbeast novel sounds interesting (and, yeah, I'll probably end up reading it), but I'm wondering if anyone knows of other books that are related to ageplay or such?