According to Official Guinness Records,
Christopher Paolini (USA), born 17 November 1983, is the youngest author of a bestselling book series. With sales in excess of 20 million copies as of May 2011, the Inheritance Cycle, which consists of the books Eragon, Eldest, Brisingr, and Inheritance, due for release on 8 November 2011, continues to be a firm favourite of fantasy fans the world over.
The first volume of the cycle, begun when Christopher was just 15, was originally released in 2002 by Paolini International LLC before being picked up and published the following year by Alfred A. Knopf, which went on to publish the rest of the Inheritance cycle. The first book in the series, Eragon, was made into a movie (released 2006). Only bestsellers are considered in this category. Although other, even younger authors have succeeded in publishing book series none has achieved anything approaching the phenomenal sales figures enjoyed by Christopher Paolini and so cannot be considered for the record of youngest author of a bestselling book series.
For a complete list of 2011 records, please visit 2011 Guinness Records in United States.