Must one who observes the obligatory and supererogatory acts of worship make up the fasts of the years he did not observe before reaching the age of twenty?

You must make up for every month you missed fasting after reaching the age of puberty (ihtilam). Additionally, you must feed thirty poor persons for each month as expiation for delaying the making up of the fasts. However, if your breaking of the fast was due to intercourse during the day in Ramadan, then you are liable for the expiation for violating the sanctity of fasting (freeing a slave; if you cannot find one, then fasting for two consecutive months; if you are unable to do that, then feeding sixty poor persons). If it was for reasons other than intercourse, scholars have differed on the necessity of this expiation. You should also increase your repentance and good deeds for committing a grave matter by neglecting a pillar of Islam.