How old was the narrator in "The Scarlet Ibis" when Doodle was born?

James Hurst's only short story, "The Scarlet Ibis," is about two brothers growing up on a cotton ranch in coastal North Carolina during the first part of the 20th century. The narrator (who is only referred to as "brother" during the course of the story) is six years old when his brother, William Armstrong, is born. Hurst writes in the third paragraph:

He was born when I was six and was, from the outset, a disappointment. 

William Armstrong is born with a physical disability. The narrator later nicknames his brother Doodle when the child is two years old. When Doodle is five years old the narrator takes him to Old Woman Swamp and teaches him how to walk. It is within a year from teaching Doodle to walk that the narrator pushes Doodle too far, and the young boy dies from internal bleeding. We can assume that the brother was about 12 when Doodle, about six, dies under the nightshade bush.