Kubo doesn't actually know what the lounge is for OR that, despite the UP's low standard for adulthood (which he technically qualifies for) many prior human societies viewed 14 as a little too young to be surrounded by alcohol. He just saw a guitar and had to come investigate.

"I've seen these on music videos!" because it didn't take him long to figure out what those were. "Do you play?"

He has his shamisen oh him, of course. But that has such a different sound from the bigger, six-stringed instrument.