What should a woman who has reached 49 years of age do about days she missed from previous Ramadans when she was around sixteen, due to ignorance of the necessity to make up those days?

You must repent, frequently seek Allah's forgiveness, and perform righteous deeds. You should also make up the days you missed fasting if you know their number. If you do not know, then fast until you are reasonably certain that you have made up enough.