Milan, July 11 - Eighteen-year-old goalkeeping phenom Gianluigi Donnarumma on Tuesday signed a contract extension that will keep him at AC Milan until 2021 earning six million euros per season. His younger brother Antonio also became a Milan player, signing from Greek outfit Asteras Tripolis in the deal engineered by agent Mino Raiola. The deal came after weeks in which 'Gigio', widely seen as heir to legendary Italy and Juventus keeper Gianluigi Buffon, was linked to Real Madrid, and after initial resistance to including his brother in the package. In a statement on its website, Milan said: "In the end, the Club and the goalkeeper have reached an agreement to extend Gianluigi Donnarumma's contract. "Surely positive news for AC Milan who have worked hard to renew the keeper's contract, for the player and the fans. "Therefore, Gigio's experience in the Red and Black continues. "AC Milan will be holding tight on one of the most talented players in the squad with the class of '99 who will be part of the backbone of the Red and Blacks in the new season. "Pure class and talent that will continue to defend AC Milan's goal ready to help the Rossoneri in the race for the Champions League."