what happens that forces jem to mature?

I think Jem really matures when Nathan Radley cements up the tree hole. Jem is suspicious, and when he asks Atticus about it, Atticus says the tree looks very healthy, but that Mr. Radley must have a good reason for plugging up the hole. Jem thinks on Atticus's statement and about who might be leaving the gifts. He stands out on the porch by himself for a long time. When he comes inside, Scout thinks it looks like he has been crying. Jem understands that people are not always good and the world can be a very unfair place.