How old is Leigh when he writes his third letter to Mr. Henshaw on November 13?

Leigh’s third letter is dated November 13, and Leigh is now in fourth grade; he made a diorama of Ways to Amuse a Dog, and Leigh says that his favorite of Mr. Henshaw’s characters is “Joe’s Dad because he didn’t get mad when Joe amused his dog by playing a tape of a lady singing.”