Jorge: You won’t be able to write well about the rock music of the 1960s, since you were just an infant then.
Jorge's 1st claim
You won't be able to write well about the rock music of the 1960s, since you were an infant then. Does Ruth disagree? Does she argue she would be able to write well about rock in the 60s, even though she was an infant then?
Yes! First of all she says, "Your reasoning is absurd", so we know that she objects to the conclusion drawn on the basis of the evidence (since you were an infant, can't write well).
Secondly, her final sentence is clearly pushing back against this notion?
Jorge's 2nd claim
60s rock was created by and for people who were in their teens and early 20s then. Does Ruth disagree? Does she argue that 60's rock was not created by and for people who were in their teens and early 20s at the time?
No. She doesn't speak at all to who created 60s rock or for whom 60s rock was created.
Goal
They are arguing over whether someone who wasn't in their teens / early 20s at the time can write well about 60s rock music.