Covering up in every situation for a young girl who has reached the age of seven is more beloved to us, because she has been commanded to pray. When a young girl reaches seven, she should cover her head in prayer and strive in covering herself to the best of her ability, just like an adult woman. If she does not do so, her prayer is valid, if Allah wills.
She must not neglect that under any circumstances when she reaches the age of nine. She must perform ghusl (full body ablution) for janabah (major ritual impurity) if she has had intercourse, because the Messenger of Allah, may Allah bless him and grant him peace, consummated the marriage with 'A'ishah while she was nine years old.
1289 - Ishaq narrated to us, saying: Zakariyya ibn 'Adi informed us, from Abu al-Malih, from Habib ibn Abi Marzuq, from 'A'ishah - may Allah be pleased with her - who said: "When a young girl reaches nine, she is a woman."
Chapter: One Some of Whose Body is Exposed During Prayer
It was said to Ahmad - may Allah have mercy on him -: 'A man prays and some of his body is exposed?' He did not see any need for him to repeat the prayer. Likewise, if he were wearing a single chemise and did not button it, he would not repeat the prayer. He mentioned a hadith narrated from some of them, in which he said: "Cover from us the buttocks of your imam."
It was said: 'What about the hadith of the Prophet, may Allah bless him and grant him peace, when he said to the women: "Do not lift your heads until the men lift theirs, and that is due to the tightness of their lower garments?"' He said: "Yes, but this does not contain a clear explanation (regarding the obligation of buttons)."
I said: 'Are the man and the woman equal in this regard, concerning buttons?' It was as if he inclined toward that view, but the woman must wear a head-covering (khimar).
(1) Thus it appears in the original, and perhaps the correct term is: "the lower garment (al-izar)."