It is obligatory upon the Person who has reached the age of Puberty to fast

Question :

I am a young man who has reached the age of 23 and my father encouraged me to fast when I was about 15 years old. Hence, I used to fast and some days I would not fast, because I did not know the true meaning of the fast. Then, after I reached the age of puberty and I understood more, I began to fast every blessed month of Ramadhan and I would not break the fast on any of its days, and all praise is due to Allâh. So, my quesion is: Do I have to make up for the fasts of the previous years, while knowing that I was 18 years old when I began to fast every Ramadhan?

Answer:

When a person reaches the age of 15 years, all of the responsibilities become obligatory upon him. For verily, this age is the sign of puberty. Therefore, this person who was lax about fasting, and he was considered to have reached the age of puberty, and he left off what was obligatory, he must make up for what he left off of the days of Ramadhan that passed that he did not fast. He is not excused because of his ignorance of the wisdom of fasting. Therefore, he must make up for the days that he left off or in which he did not complete the fast, along with making expiation for each day by feeding a poor person. If he does not know the number of days, then he must make up a safe estimate (of days) until he is sure that he has made up for what was obligatory as a responsibility upon him. And Allah knows best.

Source:
Ash-Shaykh Ibn Jibreen
Fatawa Islamiyah, Vol. 3 Pages 241-242