"The Prophet wrote the (marriage contract) with 'Aisha while she was six years old and consummated his marriage with her while she was nine years old and she remained with him for nine years (i.e. till his death)." -- Bukhari 7.62.88
In Islam, Muhammad is the supreme example of conduct (cf. Qur'an 33:21). Thus he is exemplary even in his marriage to a child: Muslims take this seriously and imitate Muhammad in this. Article 1041 of the Civil Code of the Islamic Republic of Iran states that girls can be engaged before the age of nine, and married at nine: "Marriage before puberty (nine full lunar years for girls) is prohibited. Marriage contracted before reaching puberty with the permission of the guardian is valid provided that the interests of the ward are duly observed."
The Ayatollah Khomeini himself married a ten-year-old girl when he was twenty-eight. Khomeini called marriage to a prepubescent girl "a divine blessing," and advised the faithful: "Do your best to ensure that your daughters do not see their first blood in your house."
"Saudi mufti okays marriage for 10 year old girls," World Observer, April 21 (thanks to The Religion of Peace):
Saudi Arabia's Grand Mufti okayed marriage for girls starting at age 10 and criticized those who want to raise the legal marriageable age, according to news reports. Sheikh Abdul-Aziz Al al-Sheikh said a girl becomes ready for marriage at 10 or 12 according to Islam and stressed that Islamic law is not by any means oppressive to women, the London-based al-Hayat reported Wednesday.
"Those who call for raising the age of marriage to 25 are absolutely mistaken," al-Sheikh said in a lecture he gave at the faculty housing mosque of Imam Mohamed bin Saud Islamic University in Riyadh.
"Our mothers and grandmothers got married when they were barely 12. Good upbringing makes a girl ready to perform all marital duties at that age."
Al-Sheikh's statements came in response to a question from a female attendee about marrying minor girls without their consent.
The Saudi National Human Rights Association (NHRA) has criticized the prevalence of the marriage of minors in the kingdom and considered it a violation of childhood. The association has sought to work with authorities to curb the practice and protect children's rights.
NHRA stressed that underage marriages are also a breach of several United Nations treaties including the Child Rights Treaty, which Saudi signed in 1996 and the Treaty for the Rights of Women that the kingdom joined in 2000.
Last year a similar fatwa, or religious ruling, by Salafi preacher Sheikh Mohamed al-Maghrawi allowing girls as young as nine to marry was condemned by Morocco’s Supreme Scientific Council. The council said it lacked religious validity because it was based on only one case—the marriage of Prophet Mohamed (pbuh) to Aisha bint Abu-Bakr.
But isn't Muhammad's example normative, as per Qur'an 33:21?