What is incumbent upon one who broke her fast for days in Ramadan due to a legitimate Shar’i excuse, when she was 15 years old, and did not make them up until she reached the age of 19? And does anything remain upon her after making up those days?

If a woman misses days in Ramadan due to menstruation, she is obligated to make them up. It is not required to make them up immediately; rather, it is permissible to delay until the next Ramadan. However, if the next Ramadan arrives without her having made up the days, then she must make them up along with an expiation of feeding a poor person for each day, if the delay was without a valid excuse. Since you have already made up the days, you are now only required to pay the expiation if your delay was without a valid excuse.