All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The basic principle is that there is nothing in Shari‘ah that stipulates a particular age of marriage for the man or the woman.
The issue in question, which is related to specifying a particular age for marriage, falls under a general matter, which is: Does the guardian have the right to restrict what is permissible for the public interest? This is one of the matters of ijtihad, so no one should disapprove of it for what his ijtihad has led him to.
A resolution of the Islamic Fiqh Academy on the marriage of young girls reads:
* The age of marriage for young girls is determined with the permission of the judge and the matter is left (entrusted) to the ruler in each country according to the circumstances of time, place, and age, in a way that achieves the interest of all.
* Each country has the right to determine the suitable age for marriage, according to what it deems to achieve the interest of the girl, the family, and society, and it has the right to determine an appropriate penalty for those who give in marriage a young girl without the permission of the judge.
Allah knows best.