Must I make up the days I did not fast, given that I am not certain I had reached the age of religious accountability at that time?

The fundamental principle is that one's conscience is absolved from religious obligations. Uncertainty about whether the days you missed fasting were before or after puberty nullifies the requirement for making them up, because "what oscillates between being obligatory and non-obligatory is not obligatory, as the fundamental principle is the absolution of one's conscience until the obligation is ascertained." Consequently, your conscience is absolved due to the lack of certainty regarding the obligation.