All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is permissible to propose to a young girl, even a baby girl, and her father has the right to unite her in wedlock, but her husband is not allowed to have sexual intercourse with her until she is mature. In the collections of Al-Bukhari and Muslim, it is narrated from 'Aa'ishah, may Allaah pleased with her, who said that "the Messenger of Allaah joined me in marriage when I was seven years old and consummated the marriage with me when I was nine years old." This is the wording of Muslim.
According to biographers like Ibn 'Abd al-Baarr in his book Al-Istee'ab and Ibn Hajar in Al-Isaabah, Umar ibn Al-Khattaab proposed to Umm Kulthum, the daughter of 'Ali but 'Ali replied, "She is still young." Then Umar proposed to her again, whereupon Ali said, "I will send for her and if she accepts, she is your wife." Then, Ali sent her to Umar who unveiled her leg. She said, "Stop this! If you were not the Leader of the Believers, I would beat you."
Thus, Umar accepted her as his wife and no narration stated her exact age. However, there is a discontinuity in the chain of narration and thus it is inauthentic as stated by Shaykh Al-Albaani .
Allaah Knows best.