Rambam Naarah Betulah Chapter One
...When, however, a woman who is raped or her father do not desire that she marry the rapist, they have that prerogative. [In such an instance,] he must pay the fine and depart. If she and her father desire [that the marriage take place], but he does not desire, we force him to marry her, aside from paying the fine, as Deuteronomy 22:29 states: "He must take [the maiden] as his wife"; this is a positive commandment.
Even if the girl is lame, blind, or afflicted with leprosy, he is forced to marry her and he may never take the initiative in divorcing her, as [the above verse continues]: "He may not send her away as long as he lives." This is a negative commandment.
...At what age is a girl fit to be paid a fine? From the age of three until she reaches the age of bagrut. If a man engages in relations with a girl less than three years old, the relations are not significant. If he engages in relations with her after she reaches the age of bagrut, he is not fined. For Deuteronomy 22:28 states: "A virgin maiden," thus excluding a girl who has reached maturity.
what is the reason of the law that if she was raped after she was 12 and a half the rapist is not obligated to marry her?