It's tricky, because you basically have three different usages of the term. (As proven by this thread!)
You can say "Classic Rock" as a movement, where it specifically is only 60s and 70s (-ish) rock (although modern bands can be "Classic Rock influenced" in this sense);
You can say "Classic Rock" as a genre, which is a vague label for "non-depressing/non-angry guitar-focused music with solos" of any decade, which is how a lot of classic rock radio stations are choosing to expand the scope of what they play without being forced to relabel themselves as no longer being Classic Rock;
And then you can just say "classic rock" (lowercase) as a description of something having a certain level of quality, saying "oh man, that's a classic." But this use only confuses things further, and we should try to ignore it.
Myself, I'm okay with different uses existing, as long as people understand that they're describing slightly different things-- and, they realize that none of them merely mean "old rock music of all genres." Old Grunge and Nu Metal songs are not automatically Classic Rock, no matter how good they are. (Although some songs can be kind of half-and-half to the point where they'll play as both.)
...So whatever, I don't fucking know, I voted for the first thing.