As far as I remember, I used to fast in Ramadan and would sometimes break it for some days. But I am not sure whether I was already an adult when I did so or not.

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.

A Muslim, according to principle, is free of liability unless there is an occurrence that dismisses such a fact. That you are not certain whether or not you had reached the age of puberty when you missed those days of fasting is enough to exempt you from making up for those days. This opinion is supported by the rule stated in Anwaar Al-Burooq fi Anwaa’ al-Furooq, which reads: “Being in doubt about the existence of the cause that renders something obligatory is, in itself, a reason for not considering such a thing obligatory.”

To clarify, since you are not sure whether you had missed the fast of those days before or after having reached puberty, you are uncertain about the existence of the reason based on which you would have been required to fast those days, which is reaching puberty, so you are not liable for making up such days.

Allaah Knows best.