"The most important fact right now in the lives of American teen-age boys," LIFE magazine informed its readers in a June 1945 cover story, "is that they may have to go and fight Japan. They have responded to this stern prospect by behaving exactly as they have always behaved, devoting themselves to all the vastly important details connected with the complete enjoyment of playing, eating, and sleeping, the doubtful enjoyment of exploring the world of men and women."
The article in that long-ago issue of LIFE -- titled, simply, "Teen-Age Boys" -- featured photographs by the incomparable Nina Leen, and came a half-year after the magazine's first foray into the strange universe of teens: a December 1944 article called, "Teen-Age Girls: They Live in a Wonderful World of Their Own." (See LIFE.com's "The Invention of Teenagers: LIFE and the Triumph of Youth Culture.")
That feature was shot by the same photographer who so neatly captured the teenage boys -- offering more evidence that, among LIFE's enormously talented and versatile photographers of the 1940s, perhaps none were as adept at capturing the fluid, emotionally charged lives of teens with quite the same cool sensitivity as the Russian-born Leen.
Of the (all-white) male teens Nina Leen photographed in Des Moines, Iowa, in 1945, LIFE went on to wryly observe that, among the boys, "the old skills are still admired - -"
The ability to swim well, to memorize names of football heroes, to have a quick wisecrack for the day's every small event, to be popular. The ancient foibles are still pursued -- homework is done in ten minutes, Mother is looked upon as a lovable servant, home is only for eating and sleeping. Their greatest talent is for endless happy skylarking. Their talk of girls, their cautious smoking, their cocksure arguments still go on.
The war that shadowed all of these photos -- for LIFE's readers, and for the boys in the photographs themselves -- lasted another two months, until American planes dropped bombs on two Japanese cities that, one suspects, few if any residents of 1945 Des Moines, Iowa, had ever heard of: Nagasaki and Hiroshima.
Liz Ronk, who edited this gallery, is the Photo Editor for LIFE.com. Follow her on Twitter @lizabethronk.