The narrator in the story is actually Doodle's big brother as and adult telling the story. The story begins using adult language and syntax, but as he slips into telling us about his childhood memories of Doodle he begins to take on a childlike persona and the mood and tone of the story change. The begins to sound very much like a child is telling it about his younger brother at the time things are happening. This is actually a very clever way of writing the story because we get some insights from the adult narrator that we might not have gotten had the story actually been told by the child himself. For example, at several different places in the story Doodle's brother expresses his remorse for the way he treats Doodle, like leaving him in the loft knowing Doodle couldn't climb down the ladder without help. The narrator recalls how he felt at the time and how he felt after having heard Doodle's screams.
Old enough to be an independant big brother, but young enough to not know the limitations of youth. The narrator is able to ignore the warnings of the doctors and his parents regarding Doodle because he doesn't have the life experiences to tell him what's dangerous and what's not. He's still naive enough to think everything will always be ok. But we also know that he's old enough to be trusted in caring for his physical handicapped little brother. He takes Doodle everywhere and is even encouraged by his parents to take Doodle as a tag-a-long. I guess his exact age is really up to you, but those would be some good guidelines to follow.
the narrarator is 6 years older than doodle. it says so on page 316.
The narrator in "The Scarlet Ibis" is six years old