I believe it is the final sentence: I left for college when I was 18 (that was also the age at which I was married). This is because the parentheses also give information about another incident that took place at the time - his/her marriage.
Answer:
They show where quotations are from
There’s no way anyone could anwser this we don’t know what the passage was about and we can’t see it. Next time put more info
