Islamic duties are not obligatory for a boy until after he reaches puberty, because the Prophet (blessings and peace of Allah be upon him) said: The Pen has been lifted from three: from the insane one who has lost his mind until he comes to his senses; from the sleeper until he wakes up; and from the child until he reaches puberty.
Narrated by Abu Dawood, 4399; classed as saheeh by al-Albaani in Saheeh Sunan Abi Dawood.
The sign of puberty for males is when one of three things happen: emission of maniy (semen), growth of coarse hair around the private part, or reaching the age of fifteen.
In the case of females, puberty is reached when one of these three signs appear; in addition there is a fourth sign, namely menstruation.
It is not essential that all of the signs appear; rather the appearance of one of them is sufficient to rule that the individual has reached puberty.
It should be noted that the years that count with regard to reaching puberty are hijri years (Islamic lunar calendar). If your age at the time you mentioned was fourteen years according to the Gregorian calendar, then your age according to the Hijri calendar will be approximately half a year more than that; in this case it is likely that you had reached the age of accountability.
All of this applies so long as the other signs had not appeared in your case.
Based on that, if you broke the fast in Ramadan when you had not yet reached puberty, then you do not have to do anything, because fasting was not obligatory for you (at that time).
But if you broke the fast during the day in Ramadan when you had reached puberty at that time according to one of the signs of puberty mentioned above, then what you have to do is repent and regret it, and resolve not to go back to that grave sin again.
With regard to making up the fasts, if you broke the fast during the day after having started to fast it, then you have to make up the fast, but if you did not fast at all, then you do not have to make it up and it is sufficient to repent sincerely, in sha Allah.
You have to do a lot of voluntary fasts, because that will make up for the shortcomings that occurred with regard to the obligatory fast.
Shaykh Ibn Uthaymeen (may Allah have mercy on him) was asked about the ruling on breaking the fast during the day in Ramadan without any excuse.
Breaking the fast during the day in Ramadan without any excuse is one of the gravest of major sins, and the individual becomes a faasiq (rebellious evildoer) thereby. He has to repent to Allah and make up that day on which he broke the fast. In other words, if he fasted and during the day he broke the fast with no excuse, then he has sinned. He has to make up that day on which he broke the fast, because when he started to fast it became binding upon him; because he started it on the basis that it was obligatory, he has to make it up, like a vow that must be fulfilled. But if he deliberately did not fast at all with no excuse, then the more correct view is that he does not have to make it up, because it will not benefit him at all, as it will never be accepted from him. The basic principle with regard to the act of worship that are connected to a specific time is that if it is delayed until that specific time has ended with no excuse, it will not be accepted, because the Prophet (blessings and peace of Allah be upon him) said: Whoever does an action that is not part of this matter of ours will have it rejected. And because he has transgressed the limits set by Allah, and transgressing the limits set by Allah is wrongdoing, and good deeds are not accepted from the wrongdoer. Allah, may He be exalted, says (interpretation of the meaning):
And whoever transgresses the limits ordained by Allah, then such are the Zalimoon (wrong-doers, etc.)
And just as if he did this act of worship before the time for it began, it would not be accepted from him, by the same token if he does it after the time for it has ended it will not be accepted from him unless he had an excuse.
End quote from Majmoo Fataawa ash-Shaykh Ibn Uthaymeen, 19/89
In the case that it is obligatory to make it up, if the individual has forgotten the number of days on which he broke the fast during Ramadan, then he must do what he thinks is most likely to be the case, because Allah, may He be exalted, says (interpretation of the meaning): So keep your duty to Allah and fear Him as much as you can [at-Taghaabun 64:16]. If he thinks it most likely that he broke the fast on ten days, then he should make up those ten days.
And Allah knows best.
Reproduced from Islam QA