his winning kite to his father, Baba. The incident occurs after a kite fight, and Hassan bravely refuses to give up the kite to Assef. Suffering from guilt and aware that Hassan is more courageous than he is, Amir later plants money under Hassan's mattress to make it look like he stole it. Hassan and his father later leave the hut where they live outside Amir's house, and Amir suffers guilt forever after.

In addition to this incident, Amir is formed by being privileged. He receives a fine education and can read and write, unlike Hassan. Amir is from the Pashtun ethnic group, who are favored in Afghanistan over the Hazara. As a result, Amir lives in a large and luxurious house, while Hassan lives in a mud hut outside Amir's house. In addition, Amir is the legitimate son of Baba, while Hassan is Baba's illegitimate son (which Amir does not realize until later). Therefore, Amir has a protected position in Afghan society, while Hassan does not.

The traumatic event that took place when Amir was twelve years old was when he witnessed his best friend Hassan being raped by Assad. Unfortunately, Amir neglected to intervene and help Hassan. For the remainder of Amir's life, he has to live with the guilt of not helping his friend fend off Assad. Many of Amir's significant life choices were a result of him witnessing Hassan's rape. He purposefully ruins his friendship with Hassan and struggles to come to terms with his actions as a grown adult. Decades later, Amir decides to risk his life by entering Afghanistan to save Sohrab, Hassan's son, in order to atone for his past mistakes. Amir's life has also been greatly affected by the political instability in Afghanistan, initially by the Russians and then by the Taliban. Amir and his father were forced to move to America in order to flee their Russian occupied nation. Years later, Amir returns to a dangerous Afghanistan, which is under Taliban control. Overall, Amir is a creative, traumatized, sensitive boy, who grows up into a guilt-ridden man. However, Amir finally redeems himself by saving and adopting Sohrab by the end of the novel.

This is a pivotal question, because as the novel says what happened to Amir when he was twelve truly has shaped his life. 

What Amir is referring to is the brutal assault and rape of Hassan, his best friend and the loyal servant boy of his family. Amir witnessed the attack, but he was too afraid to do anything about it. He tried to forget it, but the truth of the matter is that this act was seared into his mind indelibly. Furthermore, his act of cowardice and Hassan's unbeleivable loyalty exacerbated his guilt to a higher level. 

As an adult he still remembers what happened, and so even though he is in America, far removed from Afghanistan, he needs redemption. So, when there is an opportunity for him to go back and set things right by risking his life for Sohrab, Hassan's son, he takes the chance. Here is what he says:

"That was a long time ago, but it’s wrong what they say about the past, I’ve learned, about how you can bury it. Because the past claws its way out. Looking back now, I realize I have been peeking into that deserted alley for the last twenty-six years."

When he is beat up, he says something remarkable. 

"My body was broken—just how badly I wouldn’t find out until later—but I felt healed. Healed at last. I laughed."

The reason why he says it is because he is reliving what he should have done when he was twelve.

Amir is the protagonist of the novel, The Kite Runner.  Amir's life took a dramatic turn when two things happened: First, he failed to rush to the aid of his servant/friend, Hassan, when he was raped by the bully, Assef. Instead, he timidly hid behind a fence and watched the event unfold; later, he pretended to know nothing about the act. Then, showing his extreme jealousy of his father's loving treatment of Hassan, Amir placed a watch and money--both birthday presents--under Hassan's mattress to set him up as a thief. When Baba found out, he forgave Hassan (secretly, his son), but Hassan's father, Ali, decided to leave their home because of the seemingly disgraceful actions of his son. 

These two acts of disgraceful deceit and cowardism hung over Amir for the rest of his life.

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....

The great English poet William Wordsworth once famously said that "the child is father to the man," meaning that the experiences of one's formative years strongly influence how one's later life turns out. Amir's opening words in The Kite Runner are a prime example of this. The terrible sight that he witnesses at the age of twelve—the brutal sexual assault of his father's servant, Hassan—marks him for life.

And not only the actual event itself is significant in this regard, but Amir's reaction to it as well. For Amir, to his eventual shame, uses Hassan's rape as an opportunity to get him out of his father's life, thus allowing him to reclaim the place in Baba's affections which he believes is rightfully his.

As he moves into adulthood, Amir knows that what he did that terrible day was completely wrong, a sign of his selfishness and insecurity. So he determines that he will atone for his sins and heads off to war-torn Afghanistan to make amends. Even though Hassan is dead, he remains a hidden presence in Amir's life, inspiring him to take enormous risks to rescue Sohrab—Hassan's son—from a miserable life under the Taliban.

You're definitely on the right track.  The violent assault on Hassan, and Amir's witnessing it, certainly shapes who Amir is as an adult.  The themes concerning redemption are quite strong in this novel, and the root of Amir's quest for redemption can be traced back to this event.  As an adult, Amir at first seems to repress his memories concerning his childhood, allowing his American lifestyle to mask his guilt.  But once he discovers there is a chance "to be good again," all his decisions (to go to Afghanistan, to risk his life for Sohrab, to consequently adopt Sohrad) are based on the attack he witnesses as a twelve year old boy, a boy who could not, was not equipped to, process it at the time, but who as an adult seeks out redemption.