I think that the moment of the rape is what causes Hassan to age at a faster pace than Amir. Prior to the rape, Hassan is loyal as can be and displays trust towards everyone. It is for this reason that he believes Amir's stories. Yet, when he is raped by Assef, while others including Amir watch, it is a moment where his life permanently changed. The trust he had in others, especially in Amir, was gone in that moment. The belief that friendship and loyalty can transcend all forces was dispelled in this moment in the alley. While he still displays trust and zealous loyalty to Amir, there can be little to take away the forlorn pain he experienced as he was sodomized in the alley with no one to help him. At the same time, there is no one with whom he can tell his trauma, causing the once open and happy child to become a young man with an awful and horrific secret in his heart and psyche.