From http://en.wikipedia.org/wiki/Oldskool:

• In demoscene, an old school production means a production made before the mid-1990s.

• In computer and video games, the dividing line for Old School is often before the

  advent of 3D graphics, generally the 8 to 32-bit sprite based games of the late-1970s to mid-1990s such as Super Mario Bros.

See also: Demoscene