What is incumbent upon one who masturbated during the daytime in Ramadan, when he was thirteen years old, knowing the act was forbidden but unaware of expiation, and then made up that day in the month of Shawwal?

If you exhibit one of the signs of puberty, such as the emission of semen, then you must repent for violating the sanctity of fasting in Ramadan (definition) and for practicing masturbation, which is forbidden and invalidates the fast. According to the preponderant opinion, you are not required to offer expiation (kaffarah (definition)). However, if you have not reached puberty and no semen was emitted from you, then there is nothing upon you.