He famously wears a hoodie, jeans and sneakers, and he was born the year Apple introduced the Macintosh. But Mark Zuckerberg is no boy-CEO.At 28, Zuckerberg is exactly half the age of the average S&P 500 CEO, according to executive search firm Spencer Stuart. Once Facebook begins selling stock, Zuckerberg will be expected to please a host of new stakeholders, including Wall Street investment firms, hedge funds and pension funds who will pressure him to keep the company growing.Young as he may seem —especially in that hooded sweatshirt— Zuckerberg will be about the same age as Michael Dell and older than Steve Jobs when those two took their companies, Dell Inc. and Apple Inc., public. In his years as Facebook’s CEO he’s met world leaders, rode a bull in Vietnam while on vacation, started learning Mandarin Chinese and as a personal challenge, wore a tie for the better part of a year.Facebook, of course, got its start in Zuckerberg’s messy Harvard dorm room in early 2004. Known as Thefacebook.com back in those days, the site was created to help Harvard students — and later other college students — connect with one another online. The scrappy website later grew to include high-schoolers, then anyone else with an Internet connection. Today more than 900 million people log in at least once a month, making Facebook the world’s definitive social network.All along, Zuckerberg has shown a maturity beyond his years. As the site grew rapidly and caught the eye of big media and rival Internet companies, Zuckerberg consistently rebuffed mouth-watering buyout offers, including from Google Inc. and Yahoo Inc.Zuckerberg, who lives in Palo Alto, Calif. with his girlfriend and a white Hungarian Puli dog named Beast, has matured as a leader with the help of experienced mentors. Rebecca Lieb, analyst at the Altimeter Group, says Zuckerberg has assembled a team of "truly exceptional lieutenants.” David Ebersman, Facebook’s chief financial officer, who hails from biotech firm Genentech, is another example. Zuckerberg hired him in 2009, saying that Ebersman’s previous job, helping to scale the finance organization of the fast-growing biotech company "will be important to Facebook.”When Jobs died last October, Zuckerberg wrote on his Facebook page, "Steve, thank you for being a mentor and a friend. Thanks for showing that what you build can change the world. I will miss you.”

AP