The obvious answer is that books for adults are generally more complex than books for children. They use a wider vocabulary and more sophisticated language, deal in “adult” concepts and experiences, are fluent in abstract ideas and thoughts, and assume a familiarity with literary genres and devices that cannot be counted on in the average child reader.
Once we look carefully at this list, however, some of its items appear rather less solid. First, not all books for adults are in fact particularly sophisticated. Literary fiction of the kind that makes the Man Booker shortlist represents only a small percentage of the adult fiction published and sold, and it would misleading to take Hilary Mantel and her peers as representative of “adult fiction”. Moreover, if the vocabulary of (some) children’s books is limited, this need not imply simplicity: ask Hemingway or William Blake. Nor are sophisticated post-modern devices such as intertextuality, frame-breaking, genre-mixing and mise en abyme the preserve of adult literature: in fact, they are probably found more often in picture books for young children, from Lauren Child’s Who’s Afraid of the Big Bad Book to the Ahlbergs’ Jolly Postman.
It’s true that children’s books don’t generally deal with specifically adult experiences such as old age or marital infidelity (although some do); but equally, adult books don’t in general deal with the specific experiences of children, such as going to school for the first time. None of these experiences is more, or less, deserving of treatment in fiction than the others.
What about plots, though? Are the plots of adult books more complex than those of children’s books? Here I’m reminded of an article written by Diana Wynne Jones shortly after she started writing adult fiction in the early 1990s, having already been a children’s writer for almost twenty years. She explains that her assumptions were in fact the opposite – that a point she would have explained only once in a book for children she felt the need to repeat several times for adult readers: “These poor adults are never going to understand this; I must explain it to them twice more and then remind them again later in different terms.” This idea derived from her experience of being told by adults that they found the plots of some of her children’s books hard to follow (and that therefore they must be "too difficult for children"). Children themselves, however, never seemed to have any difficulty. Jones’s explanation is an interesting one:
Children are used to making an effort to understand. They are asked for this effort every hour of every school day and, though they may not make the effort willingly, they at least expect it.
Adults, by contrast, are used to knowing things already, and their tolerance for uncertainty – negative capability would be a good term, if Keats hadn’t already nabbed it – is correspondingly less. All of us, when we read a novel, will encounter unfamiliar ideas and unexplained facts. I suppose we must have a kind of mental “holding pen” in which to place such items, in the hope that they will be clarified and resolved at some later point. But perhaps children’s holding pens have a greater capacity than those of adults, simply because they are more accustomed to dealing with new experiences? If so, we might expect them to be more able to deal with complex plots – and, in that sense at least, to be more sophisticated readers.
I don’t think that’s a complete answer to the rather silly question with which I started – because of course complexity is multifaceted – but I do find it an intriguing idea. In any case, if I ever see an adult book with as complex a plot as Jones’s Hexwood I'll be very surprised.