Islam sets the age of marriage at puberty, as it is the natural dividing line between Childhood and adulthood. Menstruation indicates that a young girl has reached Childbearing age. This age may vary from country to country, but it is discernible and not arbitrary.
Islamic Law (Shari’ah) allows for a marriage contract (Nikah) to be drafted years before the marriage itself is actually enacted. In other words, the marriage contract is drawn up, but the contract is not executed until a later date. So even though the marriage contract can be drafted, the girl will not be “handed over” to the husband until many years afterwards. In other words, a father can marry his immature daughter off to a man before she comes of age, but the husband may not consummate the marriage until after she attains maturity.
Under Islamic Law, there are certain shuroot an-nifaadh (conditions required for the execution of the contract): for consummation of marriage, one of these conditions is that both parties are mature enough for marriage. If this condition is not met, then the marriage contract remains Mauqoof (suspended) and has no actual practical effect, i.e. the consummation of marriage is delayed until the girl becomes mature enough for that. In the example of Prophet Muhammad صلى الله عليه وسلم(peace be upon him) and Aisha (peace be upon her), the marriage contract was signed when she was immature, but only took effect until after she attained maturity. This is why Aisha (peace be upon her) remained in her father’s house for three years after the marriage contract was drafted.
Under Islamic Law (Shari’ah)—like Jewish and Christian Law—marriages are sealed after they are consummated (i.e. when the couple has sexual relations). Some Muslims think that the minimum age for marriage under Islamic Law is either nine years of age or menarche (onset of menses). But this is not correct; in fact, Islam sets no minimum age limit.
Rather, Islamic Law follows this simple dictum:
A man may have sex with his wife when she becomes sexually mature enough such that she is not harmed from having sex in any way whatsoever.
This is actually the most beautiful and all-encompassing rule of all. The Islamic scholars agree—by consensus (Ijma)—with the above stated dictum. In other words, the only hard-and-fast rule with marriage is that a man may have sex with his wife so long as she is sexually mature enough that it does not harm her in any way whatsoever.