Welcome Jenni. Impressive first post.
I am in agreement with most everyone. I saw this post this morning before I left for work, and I thought about it off and on most of the day.
To me, 'Classic Rock' is roughly the same span as everyone else's, only I think it really started with the British Invasion. Not to say the British Invasion is soley responsible, just that that was about the time the distictive 'Classic Rock' sound emerged.
The Beatles early stuff, though, I consider oldies. Their later sound falls into the classic rock sound. That's just my opinion.
As far as the end of the classic rock era, I would say probably the late 70's, with a small thread of the sound trickling into the early 80's.
It was definitely an era with it's own sound. Then, along came disco, punk, etc, which led to more far reaching sound experimantation. Along came hair metal, techno, etc.
Classic rock is, and always will be, IMO, the best era for good, consistently quality music. One good song, end to end, one after another, on the radio.
I won't even go into the mess that's out there now.