Fasting: Child Reaching the Age of Legal Responsibility
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1541
If a child reaches the age of legal responsibility [i.e. becomes bāligh] before the time of ṣubḥ prayers in the month of Ramadan, he must fast. If a child becomes bāligh after the time of ṣubḥ prayers, the fast of that day is not obligatory for him. However, if he had made the intention to keep a recommended fast, the recommended precaution (al‑iḥtiyāṭ al‑mustaḥabb) is that he should complete it.