As a historical aside, a lot of people seem to think "old school" is a very recent term, that gamers borrowed the expression from the rap/hip-hop community (or perhaps funk.) Since a couple weeks ago, I heard my mother use the term "old school" and she knows nothing about RPGs or rap or hip-hop or funk, I kind of doubt this.
"Old school" has been around a long time as a phrase that means "the way people used to do things before there was a major change." For example, there's a book Tableau of the Dutch and Flemish Painters of the Old School from 1832, and there are are books about various religious groups like Calvinists or Baptists that have split into Old School and New School based on theological differences.
This is why I reject a definition I've seen floating around: "An old school RPG is whatever you started playing." Outside of the RPG realm, the expression has always been understood to mean something other than simple passage of time; there has to have been a significant change. I think it's significant that the people who think it *is* just about the passage of time and people feeling nostalgic for their youth are also likely to claim that there's really no difference between RPGs at all, which they then follow with "so you should play the most recent one that I play".