In Lee's To Kill a Mockingbird, does Jem ever outgrow the "never declining a dare"?
Jem does learn that some things are more important than dares.
A dare is basically an exhibition of pride. If you dare someone to do something, you are appealing to the person's pride. When Scout says that Jem has never turned down a dare, it is because his pride is more important to him than common sense.
Actually, Jem has ways to avoid inappropriate dares. When Dill says that Jem is afraid of Boo Radley, he says that he has his sister to think of. This is how Scout knows that he is actually afraid.
When he said that, I knew he was afraid. Jem had his little sister to think of the time I dared him to jump off the top of the house: “If I got killed, what’d become of you?” he asked. Then he jumped, landed unhurt, and his sense of responsibility left him until confronted by the Radley Place. (Ch. 1)
When Dill accuses him of running out on a dare, Jem tells him that they need to think about it more. It is like “making a turtle come out.” As they get older, it becomes less important to Jem that he seem tough and more important that he do the right thing. He is not afraid of Boo Radley. He is curious. He realizes that Boo Radley is just a lonely and reclusive man who wants to make contact with the children, but doesn’t know how.
Jem is one of the first to realize that the gifts in the tree are from Boo Radley, and he tells Atticus not to return the blanket that Boo leaves on Scout’s shoulders during the fire. He knows that Boo is the one who sewed up his pants and left them for him to find. Jem is aware that Boo is no monster. He is their friend.