If a girl menstruates for the first time in the second half of Ramadan, is she required to make up the fast for the whole month or just the days she did not fast after menstruating?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If a girl finds one of the signs of reaching puberty, she is required to fast from then on. The signs of reaching puberty for a male are: discharge of semen or growth of coarse pubic hair or attaining 15 years of age according to the Hijri calendar. Menstruation is an additional sign on the part of females. If the girl mentioned in the question did not reach puberty before the middle of Ramadan by having none of the signs mentioned above except after this date, this means that she is considered mature from the moment she bled for the first time and this makes fasting obligatory on her from that time. Consequently, she should break her fast while menstruating and fast the rest of the days of Ramadan. Later she is required to make up the fast for the days she missed due to menstruation, and not for the days before reaching puberty. But if she had reached puberty before that date by finding any of the signs mentioned above and she did not fast, then she is required to make up for these days as well.
Allah Knows best.