How old is India Opal Buloni in the book Because of Winn-Dixie?

In the book, Opal is 10 years old during the summer that she and her father, the Preacher, move to the town of Naomi, Florida.

We know this because she says so toward the end of Chapter 4.  She tells us that she is ten and that that is why her father should tell her ten things about her mother.  Her mother ran out on her and her father some years before and she misses her.  She wants her dad to tell her things about her mom so she can feel like she knew her better than she really does.