In this 2009 interview, with 11 minutes left, Chris said that Car Kids would be about "old people complaining about young people" and that it was meant to be a song about how when you get to be 40, or even only 30, you start to talk about how "the kids, they're different now; we weren't like that".
It's from quite a while ago, so they had been writing for this song for quite some time, which makes me think that they must use it for something at some point. Hopefully. :confused: