The age of being held responsible for fasting
Question :
When is a young lady obligated to fast?
Answer:
The young lady is obligated to fast when she reaches the age of responsibility. Puberty is attained by reaching the age of 15 years or the growth of coarse pubic hair around the private part or the ejaculation of the well-known fluid or menstruation or pregnancy. Whenever any of these things occur, she is obligated to fast, even if she is a 10-year-old girl. For verily, a large number of females begin menstruating at the age of 10 or 11 years. Yet, their families will be lax and they will think that she is a little girl and they will not make her fast. This is a mistake. For verily, when the young lady menstruates she has reached the age of womanhood and the pen of responsibility applies to her (i.e., she is held responsible as an adult). And Allah knows best.
Source:
Ash-Shaykh Ibn Jibreen
Fatawa Islamiyah, Vol. 3 Page 243