Question: A girl (started) menstuating at the age of eleven, so is fasting incumbent upon her despite the fact that her health has not been very good and she is in a state in which she does not have the ability to fast. Therefore, what is it that should be determined regarding her affair?
Answer: If the situation is as you have stated, then fasting is incumbent upon her because menstruation is a sign that (females) have reached the age of puberty and that can be at the (age) of nine or more. If she is able to fast, it is obligated on her to fulfil it during the (prescribed) time (i.e. in Ramadhaan). If she is incapable or is affected by severe difficulty, she should break the fast and it is obligated on her to make up for the (fasts she has missed) on other days when she has the ability to do that.
[Source: Fataawaa Lajnah Ad-daa’imah’ 10/145]