I hate to play the "back in my day" card, but, you know, back in my day, "old school" was everything that came before Run-DMC. And that made sense. Run-DMC is a good point to draw the line: Pre hip-hop album era and album era.

That doesn't do a lot for the subsequent 30+ years though. I guess you could wrap up the rest of the '80s and the first half of the '90s as "classic hip-hop," but I don't know how to divvy up the next 20 years.