VALENCIA. Marc Marquez has become the youngest MotoGP champion as a third-place finish at the season-ending Valencia Grand Prix secured him the 2013 world title in his rookie year.Jorge Lorenzo won the race but Marquez, 20, only had to finish fourth to become the first debut-season title-winner since American Kenny Roberts in 1978.Dani Pedrosa was second, but only Lorenzo could deny Marquez. "This is a dream that comes true,” he said. "I cannot explain what I feel.”Confirmation of the title win for Marquez sparked wild celebrations throughout the 104,000-strong crowd as the young Spanish rider wrote his name into the history books."I didn’t expect to fight for the championship [this season],” Marquez told BBC Sport. "Jorge Lorenzo did a great job but thanks to my team.” Lorenzo said: "Marquez has been a deserving champion, today is his day.”