Thanks to the movie "Knocked Up," the actor Seth Rogen became the chubby, curly-haired face of male arrested development and an unexpected flashpoint in the war of the sexes. A good percentage of American opinion was apoplectic at the notion that a pot-smoking, ambition-free loser like Mr. Rogen's slacker antihero would even hook up with a hot, put-together young woman like the television journalist played by Katherine Heigl, let alone agree to raise a child with her.

Men in their late teens and 20s have historically accomplished great things. They have started record labels and newspapers and zines and social networking sites that help other men in their teens and late 20s accomplish great things. It's telling that the most talked-about businessman in the world right now isn't Warren Buffett or Bill Gates—it's Mark Zuckerberg, a 26-year-old, scruffily dressed Jewish kid who started a cultural revolution in his dorm room and inspired a movie that just may win the Oscar for best picture.

Mr. Zuckerberg isn't the only 20-something achiever who has changed the world at an age when our fathers and grandfathers were still trying to scramble up the first few rungs of the corporate ladder. In 2005, a trio of 20-something PayPal employees named Steve Chen, Chad Hurley and Jawed Karim were too young and green to realize that you can't start putting television shows and movies and seemingly every piece of filmed entertainment from the past century online just because you think it'd be cool to share, say, a clip of Johnny Cash playing a Jimmie Rodgers number alongside Louis Armstrong.

Yet Messrs. Chen, Hurley and Karim went ahead and started YouTube so that they could share the media they loved with the entire world. YouTube proved a powerful catalyst for creativity; it gave the world Justin Bieber (but don't hold that against it!) but also improbable success stories like the Lonely Island, a comedy troupe of three guys in their 20s who parlayed making goofy homemade videos with their buddies into hit albums and gigs on "Saturday Night Live" (a show that has made television history by relying on the comic energy of several generations of ambitious 20-somethings).