A young girl had her menstrual period and she was fasting during the days of menstruation due to ignorance; what in incumbent upon her?
She must make up for the days which she fasted while she was menstruating, because fasting on the days of menstruation is not accepted, and it is invalid, even if she was ignorant, and there is no limit to the time for making up missed fasts.
And there is another question opposite to this one: A women whose period came to her when she was young and she was embarrassed to tell her family, so she was not fasting. It is incumbent upon this young girl to make up for the month which she did not fast, because when a girl begins to menstruate, she becomes legally responsible, since menstruation is one of the signs of reaching maturity.
Shaykh Muhammad bin Saalih al-`Uthaymeen