12+ for mockingbird imo. He may well not be too aware of the concept of rape, and you really need to have an awareness of the context of the book, and I don't think many primary kids really understand the treatment of black people in southern state usa at that time and now
My son was a very good reader and was given To Kill a Mockingbird aged 11 - he told recently (aged 15) that he missed a lot of the story and although he could read it he realised now that he had not fully understood it. I'd wait till 13 so its fully appreciated. Also a fairly tough subject.
Wonder is an easier read - i would give to an 11 year old and I know younger children read it but it does need some maturity to fully understand the topic.
We teach Wonder in Y7 and I think that is a great age for it plus it is a great message as they start a new school. TKAM is wasted before 13, the fact you can read a book doesn't mean it is the right book for you at that age. There is more to reading than mere comprehension.