Numakawa and Isawa are mid30s, Foreman is probably 40ish which means that the songs they grew up with were made mostly in the 80s and 90s. I know the first three from my childhood growing up in Hong Kong where the music scene was dominated by pop stars covering Japanese songs. In my opinion, the 80s were defined by disco, the 90s by ballads and the 2000s with j-pop, rnb and rock, and 2010s with pop groups.
There's a running gag seen in Volume 1's one-shot where Numakawa rents a private room underground for a day, and at the end of the chapter he sings 'Sakuranbo' where it's broadcasted on the loudspeaker to everyone.
What were some of the first j-pop songs you've heard?