The June 14 issue of The New Yorker, perhaps the premier showcase for American fiction, features a list of “20 Under 40” — that is, 20 accomplished writers under the age of 40. Many of the names are familiar: Joshua Ferris, Jonathan Safran Foer, Nell Freudenberger, Rivka Galchen, Nicole Krauss, Gary Shteyngart, ZZ Packer, Wells Tower.

The purpose of the exercise, the editors explain, is “to offer a focused look at the talent sprouting and blooming around us,” in particular the talent of these “young fiction writers who we believe are, or will be, key to their generation, . . . the ones our grandchildren and their grandchildren will read.”

It is hard to quarrel with this hopeful forecast, particularly at a moment when it’s not certain anyone’s grandchildren or great-great-grandchildren will be reading fiction at all. But the emphasis on futurity misses an essential truth about fiction writers: They often compose their best and most lasting work when they are young. “There’s something very misleading about the literary culture that looks at writers in their 30s and calls them ‘budding’ or ‘promising,’ when in fact they’re peaking,” Kazuo Ishiguro told an interviewer last year. Ishiguro (54 when he said this) added that since the age of 30 he had been haunted by the realization that most of the great novels had been written by authors under 40.

At the time, this anxiety struck some as comical, but history bears Ishiguro out. Even great novelists who endure in the collective memory as Prosperos, long seasoned in their “secret studies,” often performed their greatest magic when they were young. Flaubert was 29 when he began writing “Madame Bovary” (and was 34 when it was completed). Thomas Mann was 24 when he completed his first masterpiece, “Buddenbrooks.” Tolstoy, after a period of dissolution followed by military service, began writing “War and Peace” at age 34. Joyce, who wrote “Ulysses” in his 30s, already had two major works behind him. The late-blooming Proust, his youth idled in Paris salons, was only 37 when he began writing “Remembrance of Things Past.” Even Kafka, the 20th century’s most haunting exemplar of anguished paralysis, was 29 when he wrote “The Metamorphosis” and 31 when he began “The Trial.”