The World Federation One Stop Fiqh

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. And if a child becomes bāligh after the time of ṣubḥ prayers, the fast of that day is not obligatory on 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.