The rock from the 60s and 70s got ubiquitously considered classic rock by the early 90s, but I wouldn’t classify anything beyond the early 80s as “classic”. I think that the genre may expand with time, but the 60s and 70s were the true origins of rock. The music past that is definitely not new, but not “classic”.
I guess that makes my answer G.
Edit: From a marketing perspective, I’m about to cringe hard, music up to the mid-90s is considered classic rock. To me that seems like it’s just an easy way to keep “classic rock” more entertaining and nostalgic for the older markets, and more relevant for the younger ones.
This is a great point that it's more of a era or genera than a designator to be applied as the work ages. You have Rockabilly, Classic Rock, Grunge Rock, etc.
One of the things that makes music slightly different than durable goods, although I would have a hard time calling anything from the 90's "vintage," but it's getting close to that age.
A big factor I feel is the artists themselves touring.
If the band has a hiatus, gets back together for a last tour, they're probably classic rock. If the band has a dead member and isn't replacing them, they're probably classic rock.
It's sad that Sublime, Alice in Chains, Linkin Park, and Soundgarden are all in this classic-rock-to-be state only because each band had so much life left in them. And well...
Yeah, "classic rock" is more of a genre nowadays, rather than a certain period of time. "Modern rock" is anything that has the standard elements of rock music (i.e. distorted guitars, bass, drums), and if it sounds like the music of the "classic" era, then it's considered classic rock.
"Classic" is never an indicator of age, regardless of what type of media or whatever you're talking about. It basically just means that it is a great example of that category of thing and what makes that category unique.
I assume the sound needs to be a mainstream popular at certain point of time, then overshadowed by a newer trend.
Which made me question myself:
Is nu-metal considered as classic now?
Most Allan Holdsworth's song are so far ahead of its time, can it be considered as classic?
Also what's modern rock nowadays? I know how modern metal sounds, but completely out of the loop with modern rock. Most pop songs have some elements of hip-hop instead of rock nowadays.