When can a child start fasting?
A child is required to fast as soon as they reach the age of bulūgh (commonly translated as puberty or maturity). According to Muslim jurists, the age of bulūgh that is usually stipulated for a Muslim boy is between 14-15 years, and for a Muslim girl is between 9-13 years.
However, despite reaching the age of bulūgh, in practice young people sometimes struggle to fast if they do not possess the physical or mental ability to do so. Examples of this include where a young person resides in a part of the world that has extensive hours of day light, or where the holy month of Ramadan falls during their school/examination periods.
A child is only required to fast after he/she reaches the age of bulūgh. The age of bulūgh is determined by the physiological changes that occur in the body of an individual, such as seminal discharge for boys and menstruation for girls. However, there is no obligation to perform any duty unless the capability of enactment (wusʿ) is ascertained. If a young person between the age of bulūgh and adulthood decides to fast during their school/examination periods, then it is necessary that they seek consultation and permission from their guardians, such as parents and teacher jointly.
2. Amongst the juristic maxims (qawāʿid al-fiqhiyya) that are derived from the Quran and the traditions of the Prophet and his family, the maxim of ‘eliminating harm’ (nafī al-ḍarar) acts like a meta-legal principle. According to this maxim, causing any harm to oneself or to others is prohibited, and therefore if enacting the Sharia obligation of fasting directly or indirectly cause harm to an individual, then it is prohibited for them to fast. As such, fasting during school/examination periods may equate to causing harm to a young person whose age falls between bulūgh and adulthood - for instance, it may lower their concentration span, which in turn would adversely affect their academic performance.
Since the verse of the Quran relating to fasting states that “fasting is better for you, if only you knew.”, it is necessary that the potential level of harm is thoroughly assessed. Accordingly, in line with the verse of the Quran that states “ask those who know”, it becomes clear that the most qualified group of people who can assess and advise on the level of harm an individual aged between bulūgh and adulthood may encounter whilst fasting is their guardians. Therefore, it is necessary for young people to seek joint consultation and permission from their guardians, such as parents and teachers, in order to make an informed decision regarding the level of harm they may incur whilst fasting during the holy month of Ramadan.
 See ICCI discussion: What is the age of bulugh for boys and girls in Islam?
 See definition of adulthood https://medical-dictionary.thefreedictionary.com/adulthood
 Quran 2:184
 Quran 21:7, 16:43