My DD read it at 9. Her theatre group was going to be doing the play and so she was interested in what it was about. I said I needed to think about it but she snuck a copy and started reading it behind my back. She did come forward about halfway in and fessed to reading despite my hesitations. She didn't feel right being sneaky but she very much wanted to read the book and hoped we could discuss it together afterwards. I decided to allow it.
The rape accusation went right over her head. What she understood was that an innocent black man was accused of hitting a white girl. In fact, I don't believe they even use the word "rape." I believe they say "he took advantage of me" and bring up "a violation." Since the story worked without her understanding that aspect of the accusation or how much more dire it made the situation, I didn't feel any need to point out the truth of the matter. She was most upset about this innocent man being falsely imprisoned, about a young girl being attacked by a drunk, grown man on her way home from a school event, about poor Boo locked up in the house, ect. After reading the book, she did decide not to audition for the play and steered clear from any realistic fiction for a long while.
Personally, I was going to veto her reading the book before she went behind my back. In the end, it didn't hurt her I suppose but 9 is a little young to be disillusioned with humanity. I guess it really depends on how much you think she'll truely absord and what you think she'll do with that information.