I assume that you are referring to the final pages of the novel (since my copy ends at page 154).
I'm not sure that there is a specific symbol in the last pages that represents Bryon's growing up other than his visit to the jail. Mark is twice symbolized as a wild animal--first, he is described as a "caged lion," impatiently pacing about the waiting room; the author later refers to him as having "the eyes of a jungle animal." Mark has turned vicious during his time in the reformatory, and Bryon is fearful of the change. More than anything else, Bryon sees these drastic changes in his old friend as a sign that their friendship is over as well as their youth. Both Mark and Bryon recognize that they are no longer boys: Mark will be sent to an adult prison for his bad behavior, and Bryon will continue to wonder how he could have changed things for the better. It completes the coming of age story and how Bryon has finally transformed into a young man--capable of change and knowing right from wrong--while yearning to be young again when things seemed much easier. Meanwhile, Mark will simply grow old in a prison cell.