How old is Cassie Logan in the book Roll of Thunder, Hear My Cry?

Cassie is nine-years-old.

By the end of the school day I had decided that I would tell Mama everything before Miss Crocker had a chance to do so. From nine years of trial and error, I had learned that punishment was always less severe when I poured out the whole truth to Mama on my own before she had heard anything from anyone else.

You mutter one word of this to anybody, Lillian Jean, I said, attempting to narrow my eyes like Papa’s, ‘just one person and everybody at Jefferson Davis is gonna know who you crazy 'bout and all your other business ... and you know I know. Besides, if anybody ever did find out 'bout this fight, you'd be laughed clear up to Jackson. You here going on thirteen, getting beat up by a nine-year-old.'