Unfortunately, you asked a series of questions but are only allowed one question per posting, so I had to edit. This opening line from Khaled Hosseini's The Kite Runner is quite striking and immediately captures the reader's attention; however, it is more of a figurative truth than a reality.
"I became what I am today at the age of twelve."
No person is shaped solely based on one incident, and especially if that incident does not happen for twelve years. That would require us to believe that nothing that happened in the first twelve years of Amir's (or anyone's) life did nothing to mold him into the young boy he was at the age of twelve, and nothing that happened to Amir after the age of twelve had any impact on who he is today. Obviously Amir's statement must be read as hyperbole, which of course does not discount the significance of the event to which Amir refers.
A case could be made that everything that happened before that incident is what caused Amir to do what he did (or did not do) so long ago. We do not know what the specific event to which Amir refers is until many chapters later, but in between we get a picture of Amir as a rather selfish, unkind boy. We understand what caused it, of course, and we feel some sympathy for Amir because of it; however, we cannot excuse Amir's small cruelties toward Hassan, the boy who would have done anything for Amir out of love and devotion.
When Amir deliberately fails to even try to save Hassan from a cruel and perverted Assef, we are all changed. Hassan has lost his innocence, though his devotion to and love for Amir in unwavering. Amir has lost his soul, at least for a time, and he will suffer the guilt and shame of his sin of omission for many, many years. We, the readers, are faced with the moral dilemma of feeling sorry for Amir as his life falls apart, even though we wonder if Amir deserves our sympathy.
We must also consider everything that happened to Amir after that incident. It is foolish to presume that nothing Amir sees, hears, does, or feels has no impact on his life in the years between the incident and this statement, much later in life.
What happens when Amir is twelve does, indeed, cause him to become what his is today; however, it is what happens before that time which shaped his response that day he crouched behind a wall near an alley and did nothing. This statement by Amir is a more accurate statement about the impact of certain events in our lives:
It may be unfair, but what happens in a few days, sometimes even a single day, can change the course of a whole lifetime....