What is the ruling if one makes a vow before reaching the age of legal accountability

Question:
The questioner says: When I was in middle school, and specifically in the proficiency year, I made a vow and said word for word: "If my Lord makes me pass, I will fast every Monday and Thursday." However, I was not concerned about it and did not fulfill my vow. I am now on the verge of entering university, and I have repented to Allah and sought forgiveness for my sin, so I have started fasting on Mondays and Thursdays. Is there a sin on me for what I neglected in the past, and is there an expiation? I ask for clarification on this.

Answer:
If you had reached puberty at the time of the vow, whether by menstruation, by completing fifteen years of age, by the growth of coarse hair around the private parts, or by the emission of semen through desire via wet dreams or otherwise, then this vow is obligatory upon you. This is because a woman reaches puberty through four things: completing fifteen years of age, the emission of semen through desire at night or during the day—even if through a wet dream—the growth of coarse hair around the private parts, or menstruation. A man is the same as a woman in this regard, without exception, except for menstruation, which is a characteristic specific to women. If you had reached puberty by one of these things at the time of the vow, then you must fulfill your vow and fast on Mondays and Thursdays as you vowed. This is based on the saying of the Prophet (peace and blessings be upon him): narrated by al-Bukhari in the Sahih. And you must make up the days you neglected and did not fast.