The GKLocalPlayer class is a subclass of the GKPlayer class. It includes additional properties for the local player and methods for authenticating the local player. The GKLocalPlayer class provides the following properties:
•    friends -> contains the identifier strings for other players on Game Center that are marked as the local player’s friends.
•    underage -> states whether this player is underage. Some Game Center features are disabled when the value of this property is YES. This property is provided so that the application has the option to disable its own features for an underage player.