I read this when I was too young (under age 10) to understand it, because my older sister left the book sitting on a table in the house. The boys' 5th & 6th grade teacher uses this book as a read-aloud. My oldest son had to read this again in 9th grade at Pioneer. And this year, the youngest one read it as a 5th grader in (an adult-led) book club. After helping him keep on track with some of his weekly readings, I decided to read it again myself - and began to wonder if I'd ever really read the entire book! Perhaps I gave up back in the 3rd grade and only saw the movie. So much I didn't remember: Jem reading to Mrs Dubose; Scout serving cookies to the ladies' missionary society social hour, etc.
So now I am left to wonder what is the best time to read the book. How much really sinks in when you are 10 and 11, even with an adult helping you pull out important themes? What do you get from it as a high schooler? [That son is now in 10th grade, and had to do a mock trial for this year's English class. He chose Atticus Finch as their lawyer's name, because 'he's the best lawyer ever' or something along those lines.] Perhaps the best solution is to read it multiple times at many ages, viewing it through the lenses of your own different experiences.
This book is a critical piece of fiction in our history. It shows us an important part of history. The trial of Tom Robinson is just one of many different trials like it, and it shows how much racism has affected our country.