Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No. 4147
Fataawa Ramadhaan – Volume 1, Page 49-50, Fatwa No.16
Question: A girl reached the age of 12 or 13 years and the blessed month of Ramadhaan passed without her having fasted. Is there any sin upon her or her family?
Is it upon her to fast and if she doesn’t then is she sinning?
Response: The girl becomes Islaamically bound with [the following] conditions: Islaam, sanity and puberty. Puberty is attained either by menstruation, or the excreting of discharge as a result of sexual desire, or having a wet dream if she saw the discharge, or the growth of thick hair around the front private part, or reaching the age of 15 years. So if this girl fulfilled the conditions of being legally bound by Islaam, fasting is obligatory upon her and it is a must that she makes up the fasts she left off while being at that age. And if one of the conditions are not fulfilled then there is nothing required of her.
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad ﷺ and his family and his companions.