“That’s because you can’t hold something in your mind but a little while,” said Jem. “It’s different with grown folks, we – ”
His maddening superiority was unbearable these days. He did not want to do anything but read and go off by himself.

– Harper Lee

To Kill a Mockingbird, Chapter 14. Jem believes Scout can’t understand about Atticus having a lot on his mind with the Tom Robinson case because her mind doesn’t hold on to things for long. Jem does understand because he is one of the “grown folks.” As he gets older, he starts to treat Scout like a little kid.