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.
Ruling 1542.* If someone has been hired to keep the qaḍāʾ fasts of a dead person, or if he has to keep a kaffārah fast of his own, there is no problem in him keeping a recommended fast of his own. However, if someone has to keep his own qaḍāʾ fasts of the month Ramadan, he cannot keep a recommended fast [until he has kept his own qaḍāʾ fasts], even if he has vowed to keep that recommended fast.
In the event that he forgets and keeps a recommended fast and remembers this before ẓuhr, his recommended fast becomes invalid but he can change his intention to an intention of keeping a qaḍāʾ fast. However, if he becomes aware after ẓuhr, then based on obligatory precaution, his fast is invalid, but if he remembers after maghrib, his fast is valid.