I pretty much agree with the previous 3 posters, except I'm more inclined to believe the classic rock label is more of a corporate creation that greater than or equal to our own perception of the music. Case in point: one of the classic rock radio stations where I live has gone under a format change and rebranding...
Originally, they were 95.1 the FOX, "Classic Rock that REALLY Rocks! and played a heavy dose of the same 100 or so 70's hits from Led Zeppelin, the Stones, the Who, Lynyrd Skynyrd, etc. with some sprinkling of the Beatles and a few Ozzy songs from the early 80's.
Now, they are 95.1 the BREW, "Playing the NEXT Generation of Classic Rock!" and play a heavy does of the same 100 or so 80's hits from Bon Jovi, Foreigner, Poison, Motley Crue, etc. with a sprinkling of Led Zeppelin and a few Nirvana songs from the early 90's.
Take a look at these Super Bowl car commercials from the early 2000's and from 2012... whose being used as the music?
I couldn't find an older Cadillac commercial on youtube, unfortunately. But think of it as these car companies and radio stations (owned by giant media conglomerates) are trying to advertise to new customers... people age 28-35 who finally have some disposable income to spend on nice cars and other products. When would they have been young kids growing up... roughly around the time 'Rock and Roll' and 'Kickstart my Heart' were released.
Same idea difference product. Retirement commercials recently? Check out the music they're using...
It's all about knowing your audience and things that are "classic" are a great way to relate to others. However, despite all my cynicism about the subject, maybe since we know the classics from our childhood; these corporations are aware of our love for the music and wanting to relate to us.
Either if we choose or are told what classic rock is, if you enjoy it... that's really all that matters. 