I work for an Oldies station in a college town and I have college students all the time telling us that they listen. We're VERY heavy 70s with late 60s and early 80s in for good measure.
We also have a CHR and we'll get people coming in to pick up prizes (20-somethings) that we're sure are CHR listeners and it turns out they won on our Oldies station.
As far as the "commonality" of the music, I'd say that memory fades over time and that "commonality" will remain true with 60s/70s/80s stations for years to come. Here's an example...I was in High School in the 80s. I hated most of the music in the 80s -- especially the "Hair" bands. But, now 20-something years later, when I hear a song from the 80s, it brings back memories and I'll listen to the song.
Think about songs from the late 60s...you've got acoustic stuff like Simon & Garfunkel, Motown like the Supremes, Beatles, "hard rock" stuff like Steppenwolf. I'd argue that the music then was just as diverse as today's Top 40.
Here are the Top 10 from 1968 as an example...
1-"Judy In Disguise"-John Fred/Playboy Band
2-"Green Tambourine"-Lemon Pipers
3-"Love Is Blue"-Paul Mauriat
4-"Dock of the Bay"-Otis Redding
5-"Honey"-Bobby Goldsboro
6-"Tighten Up"-Archie Bell & The Drells
7-"Mrs. Robinson"-Simon & Garfunkel
8-"This Guy's In Love With You"-Herb Alpert
9-"Grazing In The Grass"-Hugh Masekela
10-"Hello, I Love You"-The Doors
To me, that's a pretty diverse grouping of songs. Yet, we can listen to all of those songs in an Oldies format and there's a "commonality" to them.
I'd say Oldies/Classic Hits/Classic Top 40/whatever you want to call it will be around for a long time.
In answer to the question that started this thread, Boomers are the BEST ad target. 20-somethings don't have any money and are getting harder and harder to reach. My parents are in their early 70s. They change products all the time based on advertising.
Ad agencies need to get with the program. 35-64's are ruling the world NOT 18-34s! And another thing...it's time to put the 25 - 54 demo out to pasture. A 25 year old and 54 year old are such polar opposites, it's not even funny!