I have a 16yo daughter (so not a “millennial”) and to her it’s all just “music”. Yeah, she understands that some songs are “old” but her approach to music and her sense of identity with it is far more about “here’s what I like which is a reflection of who I am” and not “here’s what I like as a means of setting myself in opposition to anybody else”.
For example, on her playlist are Celtic songs, Frank Sinatra, Ariana Grande, Mozart, One Direction (and Harry Styles, always Harry Styles), Tom Rosenthal, Harry Connick Jr, her Rockabilly phase, her “moody girl” phase, Led Zeppelin, movie soundtracks ranging from Disney Tunes to Guardian of the Galaxy 70s compilations to the Avengers soundtracks, Rhianna, Alanis Morissette, The Beatles, The Who, Benny Goodman & Glenn Miller, etc etc etc.
The kid knows more music… and knows more about music… than I ever had the chance growing up in the late 70s/early 80s, and it’s all because of YouTube. Yeah, Pandora/Spotify helped, but YouTube is the primary driver of musical tastes for her generation as it is ubiquitous, free, easily accessed, easily shared, and did I mention free? You can find, for almost any song ever, the original, the original with lyrics, the original set for karaoke, covers, instructional “how to play” videos, etc.
She and her friends spent years sharing new songs with each other, learning the lyrics, more… and they all did it via YouTube.
And it leads to surprises like that time I was listening to the oldies station with her and 2 friends in the back seat (about 12yo at the time), when Chicago’s “Hard to Say I’m Sorry” came on… and all three of them got really excited and started belting out the song as it was playing. The song was released in 1982, this happened in 2013, so it would have been the equivalent of 1979 me singing along to a 1948 hit my Dad had on the radio. Never would’ve happened. But with my kid and her friends? Totally normal.
So I am positive that Sophia knows if a song is “old”, but I am also positive it doesn’t really matter to her.