In the book Because of Winn-Dixie the young girl India Opal Buloni is 10 years old. Opal finds a big ugly dog in the Winn-Dixie Supermarket after her and she move to Florida. Her father, a preacher offers to tell Opal ten things about her mother, one thing for each year she has been alive. Her mother had left the family when Opal was only three years old and before they moved to Naomi. Florida. Prior to finding the dog and naming him Winn-Dixie the young girl has not made friends in her new town. Winn-Dixie helps Opal to make new friends.
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.