How can I make up for missed prayers, given that I do not know when I reached puberty? And should I consider the age of puberty to be fifteen or twelve, in order to determine when to begin making them up?

We have discussed that whoever doubts the onset of puberty, the default is that it has not occurred. So, if you doubt whether you reached puberty at, for example, twelve or fifteen years of age, your puberty is estimated to have occurred at fifteen, because the default is non-puberty. Therefore, you are only required to make up what you are certain was obligatory upon you, and that is from the time you became certain of puberty. This is Allah's ruling on the matter. As long as you comply with it, you are neither negligent nor deserving of punishment. Disregard intrusive thoughts and pay them no attention.