I think a prevailing theme in the section that we read was youth, and the loss of youth.  There are young characters in this book, and there are old characters.  The young characters can take their youth for granted, while the older characters mourn the loss of the times when they were young.  One such character that really personifies the idea of lost youth is Charles Halloway.   He thinks that he is too old to play with his young son, which he verbalizes when he says, "Will makes me feel so old.  A man should play baseball with his son." (pg 28)  before that, when he sees Jim and Will running together, he has to suppress "a sudden urge to run with them, to make the pack."  But the most revealing scene comes when he is looking in the mirror and sees himself as an old man.   He is the symbol of lost youth in the novel.
The characters representing youth in the book are Jim and Will.  They are both young.  Though Jim is less naïve, he is still a young boy.  Will is more innocent that Jim, though he begins to lose the innocence when he realizes that one day, he too will be old, just like his father:  "Dad, thought Will, why, why, he looks...like me in a smashed mirror!"  He looks at his father differently after that.