Opal is 10 (well, 11 when she tells the story). She's willing to use her own toothbrush on a stray dog. She likes hanging out with old people. Sounds quirky, eh? Yep. And that's what makes her voice so appealing.
She tells the story as a flashback about "last summer," so we know she's 11 (1.1). By telling it the summer after it happened, she also clues you in on events that are a Big Deal. This helps us see the story as a narrative rather than a sequence of unconnected events. As she says, "Just about everything that happened to me that summer happened because of Winn-Dixie. For instance, without him, I would never have met Gloria Dump" (9.1).
The other thing about Opal is that she's 11. This means she still sees things as a kid would see them and speaks with imperfect grammar (gasp!).
Let's look at an example. While giving Winn-Dixie a bath, she says, "I could tell he didn't like it. He looked insulted, and the whole time, he didn't show me his teeth or wag his tail once. After he was all washed and dried, I brushed him good. I used my own hairbrush and worked real hard at all the knots and patches of fur stuck together" (3.1).
Oh, those pesky adverbs—we can't help ourselves—good should be well, and real should be really. Whew! That feels better. But we can't complain. In the end, it feels like you're both walking down the streets of Naomi as she talks, waving to her friends and patting Winn-Dixie's big, shaggy head.