Under Sharia law girls can marry at the age of 6, just like the Prophet Muhammad from the Koran did when he married Aisha. Muhammad consummated his marriage with Aisha when she was nine.
Here is a proof quote from the Islamic scriptures (Hadiths) that Muslims consider authentic record that Muhammad’s favorite wife, Aisha, was six when Muhammad wedded her and nine when he consummated the marriage:
“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).