I swear I just got slapped by Black Swan Green when I read that line in Sag Harbor, when Melanie and Benji are reminiscing about how things were when they were just kids. Melanie then asks Benji "What happened to you?". Just like Holly (?) asked Jason towards the end of his story. Which I find really interesting. In both cases, these characters kind of pop up towards the end of the book, presumably towards the end of the character's developmental arc, and asks "what happened to you?". This could, of course, be a ploy to get the reader to really think about what they just read for the last 3/4 of the book, but it could also be something to force the character themselves to reflect. What did happen to them? Did they really change?
I also think it's interesting that in both cases, the questioning character is a girl. A girl who shows up later and is nice to our main character, who seems to understand him in a way that no one else has yet. Not to say that she's perfect and understands everything about him, but she understands him and sympathizes with him. I don't really know what to make of the fact that they're girls who are insightful - maybe it's the authors acknowledging the girls' sensitivity to matters, maybe it's the authors recognizing that the entrance of girls into the boys' worlds brings something new and more intimate in a non-romantic or sexual way that the boys don't seem to really think about.
Unrelated (or related, you decide), it's interesting how both Holly and Melanie end up 1. making the first move on their guy and 2. end up kissing the guy and 3. end up splitting up with the guy not long after due to travel circumstances.