Jem Finch is a mature and responsible son because he cares about what his father thinks of him and wants to do the right thing not to avoid punishment, but so his father will be proud of him.
Jem, Scout, and Dill are always interested in the Radley place. When Jem accidentally loses his pants on the Radley porch, he lies and says they were playing strip poker with matches. He does not want Atticus to know that he disobeyed him about leaving the Radleys alone.
Scout is younger and more immature, and does not understand Jem’s real dilemma. She is afraid that Jem is going to be shot by Nathan Radley, who has a gun and thinks there is a prowler on the loose. Jem is willing to risk it though.
Scout tries to tell him he will get in trouble when Atticus finds out, but it is better than dying. Jem tries to explain.
He blew out his breath patiently. "I- it's like this, Scout," he muttered. "Atticus ain't ever whipped me since I can remember. I wanta keep it that way." (ch 6)
Scout does not understand that Jem is not just afraid of being punished. He does not want Atticus to be disappointed in him. He wants to show that he is mature and responsible, more like an adult than a little kid. Jem is moving from childhood to adulthood.