While you have not specified which of Joyce's stories from Dubliners you are referring to, I can assume that you are referring to "An Encounter," as it is the only story to feature school-aged boys playing a game. For future reference, when referring to a collection of short stories, you should always specify the specific story to which your question regards.
In "An Encounter," the narrator tells the audience that one of his friends, Joe Dillon, introduced the spirit of the wild west to his group of friends through stories, and that they would frequently arrange make-believe battles between Cowboys and Indians. Joe, who always won, would always do a signature war dance after the game was completed. Based upon this information, we can guess that these boys are in early adolescence, aged anywhere from nine to twelve. They are young enough to still make-believe play, but old enough to play truant from school later in the story.