Beverly Cleary's story in Dear Mr. Henshaw begins when Leigh Botts is younger; however, most of the story spans his whole sixth-grade year, which places his age at about 11 or 12 throughout most of the book.
In the second letter Leigh writes to the author Mr. Henshaw, dated December 3rd, Leigh reminds Mr. Henshaw of a letter Leigh wrote the year before in the second grade. Leigh's second-grade teacher had read Ways to Amuse a Dog to the class, and Leigh liked the book so much that he wrote to the author. Therefore, we know the story opens when Leigh is in the second grade, which makes him about 6 or 7 years old. In the next three letters, Leigh transitions from the third grade to the fourth grade to the fifth grade and writes to tell Mr. Henshaw about class assignments he completed on Ways to Amuse a Dog.
In the letter dated September 20th, we learn Leigh is now in the sixth grade and must write a report on an author, so he writes to Mr. Henshaw to ask him interview questions. We also learn Leigh is now at a "new school in a different town." Along with Mr. Henshaw's answers to Leigh's questions, which come too late for Leigh to include them in his report, Mr. Henshaw asks a list of questions he wants Leigh to answer to get to know him better. The rest of the story is told based on Leigh's diary entries and answers to Mr. Henshaw's questions throughout his sixth-grade year. For example, in one of Leigh's replies, we learn Leigh "used to live in a mobile home outside of Bakersfield," but his parents recently divorced, so his parents sold the home ("November 22"). In the letter dated November 24th, we learn that, after the divorce, he and his mother moved into a very small, run-down cottage in Pacific Grove, California, where Leigh spends his sixth-grade year and stays through the remainder of the book.