Answer by taking a quote from the following article:

Irving Grant Thalberg (May 30, 1899 - September 14, 1936) was an American film producer during the early years of motion pictures. He was called "The Boy Wonder" for his youth and ability to select scripts, choose actors, gather production staff, and make profitable films, including Grand Hotel, China Seas, Camille, Mutiny on the Bounty, and The Good Earth. His films carved out an international market, "projecting a seductive image of American life brimming with vitality and rooted in democracy and personal freedom," states biographer Roland Flamini.

He found work as an office secretary at Universal Pictures' New York office, and later became personal secretary to the studio's founder and president, Carl Laemmle. Among Thalberg's duties were transcribing and editing notes that Laemmle had written during screenings of his films. He earned $25 weekly, becoming adept at making insightful observations, which impressed Laemmle.  Laemmle took Thalberg to see his Los Angeles production facility, where he spent a month watching how movie production worked. Before returning to New York, Laemmle told Thalberg to remain and "keep an eye on things for me." Two months later, Laemmle returned to California, partly to see how well Thalberg was able to handle the responsibilities he was given. Thalberg gave him suggestions, which impressed Laemmle by his ability to understand and explain problems.  Thalberg suggested, "The first thing you should do is establish a new job of studio manager and give him the responsibility of watching day-to-day operations." Laemmle immediately agreed, "All right. You're it." In shock, Thalberg replied, "I'm what?" Laemmle told him to take charge of the Los Angeles studio, which he did in early 1919. At age 20, Thalberg became responsible for immediately overseeing the nine ongoing film productions and nearly thirty scenarios then under development.  In describing the rationale for this early appointment as studio manager, film historian David Thomson writes that his new job "owed nothing to nepotism, private wealth, or experience in the film industry." He reasons that despite "Thalberg's youth, modest education, and frail appearance . . . it is clear that he had the charm, insight, and ability, or the appearance of it, to captivate the film world."  Thalberg was one among the majority of Hollywood film industry workers who migrated from the East Coast, primarily from New York. Some film actors, such as Conrad Nagel, did not like the 5-day train trip or the sudden warmth of the California climate. Neither did Marion Davies, who was not used to such "big wide spaces." Samuel Marx, a close friend of Thalberg's from New York, recalled how easily Thalberg adapted to Southern California, often standing outside his doorway during moments of contemplation to enjoy the scenery. "We were all young," said comedian Buster Keaton. "The air in California was like wine. Our business was also young--and growing like nothing ever seen before."

How did he get the job so young?
?" Laemmle told him to take charge of the Los Angeles studio,