Sheikh Ahmad al-Khalîl, professor at al-Imâm University, Qasîm Branch
If, upon reaching puberty, you knew and fully understood what the consequences were of being an adult, then you have to make up all the days you missed. You may fast these days in any way that it is easy for you. However, you should not draw it out unnecessarily.
If you did not know the legal rulings at that young age and did not really understand that you were obligated to fast upon reaching puberty, then it seems more likely that you do not have to make up the missed days of fasting.
And Allah knows best.
Source: Islam Today