who do you think charlie is writing to? Are there certain age groups/people who listen better than others?

I've always believed that Charlie is writing to the readers (us), but there's always been a part of me that believes he was really writing them to himself (like a journal).

As far as the second question...... I'm going to assume you're talking about the teenagers in the novel. He definitely leans toward older friends, and I think it's not so much their maturity level or the things they do that attract him, but rather their listening skills. In the real world this is certainly true, I've often found the most attentive age group to be around five. :-D