They grabbed 1 song from the top 10 (excluding top 3 to avoid mega-hits) for every other year. Here’s the songlist used for the study:

Image

I went to the paper and if you’re struggling to interpret the x-axis like I was, the paper labels it “song-specific age” i.e. the age the person rating the song was when the song was released. Y axis is “standardized musical preference”.