I once broke fasting during Ramadan, but I am not sure as to whether or not I had reached puberty then. What is incumbent upon me to do? May Allah reward you.
All perfect praise be to Allah, the Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The signs of puberty for the male are the discharge of semen accompanied by lust, or the growth of coarse hair around the pubic area, or the attainment of fifteen Hijri years of age. The same apply to the female with the additional of menstruation. Therefore, if you are able to determine with full certainty, or at least what is close to certainty, whether or not you had reached puberty when you broke fasting, then you must do so.
If you are fully certain, or predominantly think, that you had attained puberty when you intentionally broke your fast, then making up that fast is due upon you along with repentance, for at that time you were competent to undertake religious obligations. Also, it will be due upon you to give an expiatory amount of food to an indigent person for each broken fast that was not made up for, without a valid excuse, until the coming of the next Ramadan, if you were aware of the obligation of making up before the coming of the next Ramadan.
Nevertheless, if you are fully certain, or at least predominantly think, that you had not reached puberty when you broke fasting in those days, then nothing is due upon you, for a Muslim who has not attained puberty is not competent to undertake religious obligations according to the consensus of the scholars.
If you have doubts as to whether or not you had reached puberty at that time, then the fundamental rule is being free from obligation, which means that nothing is due upon you regarding those broken fasts, for the continuation of the confirmed ruling of not having reached puberty is sufficient to exempt you from the consequences of having reached it, as nothing is due on anyone except with full certainty or at least a predominant conjecture.
Allah Knows best.