SHAFAQNA – The Grand Ayatollah Makarem Shirazi answered a question about marriages for under 13 years of age.
Question: Is marriage for under 13 years of age Haram?
The Grand Ayatollah Makarem: In the past parents were allowed to choose spouse even for minors, and usually there was no problem, but in our time it has been proved practically that these types of marriages are not beneficial to boys and girls, and are prone to misconduct; and considering that the best ways must be opted, these types of marriages are null and void. Regarding to what have been done in the past, if at that time, there was no real misconduct, and the interest of the minor has been observed, it is ruled correct.
Source: PERSIAN SHAFAQNA