Reference: Fataawa Ramadhaan – Volume 1, Page 343, Fatwa No.290
Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.3325
Question: A young girl began her menses when she was [just] eleven years old. Is she required to fast, bearing in mind that she is not well and in an unfit state to fast, so what is required of her?
Response: If the situation is as you have mentioned, then she is required to fast. This is because the onset of the menses is one of the signs of womanhood – so long as it started anywhere from nine years of age and on.
So if she is able to fast then it is obligatory upon her to do so at its appointed time, and if she is incapable [of fasting] or she experiences great difficulty in fasting, then she is to refrain [from fasting] and [in that case] she is required to make up the missed fasts when she has the ability to do so.
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.