That raises a semantic question: do “oldies” and “classic rock” refer to a simple “current year minus X to minus Y range” categorization? Or is it referring to a specific cultural moment/context, e.g. the pre-psychedelia era of pop rock, and the standard-defining era of the late 60’s to early 80’s?

Yeah exactly. In the 90s Chuck Berry was in his 60s. All the stars of the 90s are in their late 50s/early 60s now. The 90s are becoming the oldies.