I take umbrage at them being called "classic rock." Classic rock is, to me, a specific genre of rock music from the 60s–80s, and it has a particular style and sound.
Throwing Green Day and Linkin Park in with the Eagles and Fleetwood Mac just makes the word "classic" meaningless.
Yeah to me Classic Rock is 60's and 70's. Classic Rock is a genre not classic rock, rock which is classic, classic meaning it has stood the test of time. Linkin Park may be classic at this point, but they're not Classic Rock.
Old enough to remember when the first 'classic rock' stations started replacing 'oldies' stations. In those days, the 'classic' stations would play new music from old bands, and even brand new stuff from new bands. These days, a "1980's" station would never play a record from 1979 or 1990.
Because different subgenres emerged, and "rock & roll" needed clarification. The new subgenres already had adjectives. Those same adjectives apply, to-day, and they will never become original-flavor rock & roll.
I think we'd have to differentiate what's considered classic from genres themselves. Rock's been around for about 70 years, so it makes sense that a classic era emerged about 20 or 30 years after that. You could see it as something of a golden age of the music. Punk started in the 70s, so if you want to call Green Day classic punk (although no one really uses that term) I think it'd fit.
Similarly if someone said classic ska I'd be thinking of 2nd wave, like the Specials, which is also about 20 years after the start of the genre.