He fasted before he reached puberty and he forgot to make up some days. Can he make them up after reaching puberty?
Question

                When I started middle school – and I was 13 years old at that time – I broke the fast on three days and forgot about it, and I did not remember it until this year. Now I am sixteen and I have not reached puberty. Should I make up the fasts only or do I have to do anything else?.
Praise be to Allah.
    Firstly: 

    A child is not obliged to do the obligatory duties of Islam 
    until after he reaches puberty, because the Prophet  (peace and 
    blessings of Allaah be upon him) said: “The Pen has been lifted from three: 
    from the insane person who has no control over his reason until he recovers, 
    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 Abi Dawood. Whatever the child does of obligatory 
    duties or mustahabb deeds before reaching puberty, he will be rewarded for 
    them. Whoever performs Hajj when he is a child or fasts or prays, the reward 
    for that will be written for him in full, but his Hajj does not count as the 
    obligatory Hajj or Islam, and he has to do Hajj again after he grows up. 

    It was narrated from Ibn ‘Abbaas (may Allaah be pleased with 
    him) that the Prophet  (peace and blessings of Allaah be upon him) met 
    some riders in al-Rawha’ and he said: Who are these people?” They said: We 
    are Muslims. They said: Who are you? He said: “The Messenger of Allaah.” A 
    woman lifted up a child and said: Is Hajj valid for this one? He said: “Yes, 
    and you will have the reward.” Narrated by Muslim (1336). 

    Al-Nawawi (may Allaah have mercy on him) said: 

    In this report is the evidence for al-Shaafa’i, Maalik, Ahmad 
    and the majority of scholars, who said that the Hajj of a child counts and 
    is valid, and he will be rewarded for it, but it does not count as the 
    obligatory Hajj of Islam, rather it counts as a voluntary Hajj. This hadeeth 
    is clear on this issue. End quote. 

    Sharh al-Nawawi (9/99). 

    Al-Khattaabi said: 

    It counts as Hajj for him with regard to attaining the 
    reward, but it is not to be counted as his obligatory Hajj if he lives until 
    the age of puberty and reaches manhood. This is like prayer, which he (the 
    minor) should be told to do if he is able to, although it is not obligatory 
    for him, but the reward will still be written for him, by the bounty of 
    Allaah, and it will also be written for the one who tells him to pray and 
    teaches him to pray. 

    See: ‘Awn al-Ma’bood (5/110). 

    He is not enjoined to make up the fasts that he missed when 
    he was a child, and he is not to be enjoined to make up any prayers that he 
    missed. 

    Ibn Qudaamah (may Allaah have mercy on him) said: 

    As for the days that he missed (of Ramadaan) before reaching 
    puberty, he does not have to make them up, whether he fasted them or not. 
    This is the view of the majority of scholars. Al-Mughni (3/94). 

    Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have 
    mercy on him) said: 

    It is not obligatory for a child to fast, but his guardian 
    should tell him to fast so that he will get used to it. Fasting – for the 
    child who has not yet reached puberty – is Sunnah, and he will be rewarded 
    for fasting, but there is no sin on him if he does not fast. End quote. 

    Fiqh al-‘Ibaadaat (p. 186). 

    Based on this, you do not have to make up these three days 
    that you did not fast, because fasting was not obligatory for you. But if 
    you want to make them up there is nothing wrong with that. You should only 
    make up the fasts, one day for each day missed, and you do not have to do 
    anything else. 

    Secondly: 

    You say in your question that you are now 16 years old and 
    have not reached puberty. 

    But you have in fact reached puberty now, because puberty is 
    reached when one of three things happens which in the case of the male are: 

    1-Emission of maniy (semen)

    2-Growth of coarse hair around 
    the private part (pubic hair)

    3-Reaching the age of fifteen.

    In the case of the female, there is a fourth sign which is 
    the onset of menstruation. 

    When one of the signs is established, then puberty has been 
    reached. It is not essential for all of these signs to be present. 

    See also question no. 70425. 

    As you have reached the age of sixteen, you are considered to 
    be an adult, so you should act on that basis. The time of childhood and not 
    being accountable has come to an end, and the angels record everything that 
    the adult does, good or bad. 

    “So whosoever does good equal to the weight of an atom (or 
    a small ant) shall see it.

    8. And whosoever does evil equal to the weight of an atom 
    (or a small ant) shall see it”

    [al-Zalzalah 99:7-8]

    And Allaah knows best.