Novelists (and short story writers) who publish a first book in their twenties are perceived as wunderkinder. It is considered much more usual to begin publishing books in your thirties. I'm basing this on my experience as a keen reader of "literary" fiction and as a fiction writer (with a handful of pubs, but no book, dammit). And no, if I published one today, I wouldn't be considered a wunderkinder ;-)

Now, I'm pretty confident in my assertion that this is the general perception, but I haven't compiled the facts to see whether perception coincides with reality. Certainly, I can think of many young first novelists from over the years: Michael Chabon, Philip Roth, Zadie Smith, Bret Easton Ellis, David Foster Wallace. Yes, Neal Stephenson, you hackers, you. But if you check out the press and reviews on their first books, you'll find that almost the most salient fact about them is their age.

Assuming most first fiction books are published in the writer's early thirties, why would that be? Writing fiction requires a lot of disparate skills. It takes a long time to develop them, and they tend to develop asynchronously. The typical case, is a young writer with the technical skills, but not enough life experience to really get inside anyone's head, which is necessary in fiction, of course. (Certainly, it sometimes works the other way around.) Some young writers are so good at the technical aspects that the reader is willing to look past shallowness of characters, a tired theme, etc. And their books are published despite flaws. I'd say this is the case with Zadie Smith's _White Teeth_ for instance.

And of course (he says cynically) sometimes books are published for reasons other than their quality.

Murakami, BTW, has all but disavowed his first two novels.