He used not to do ghusl for janaabah out of ignorance; should he make up the prayers?
Question

                When I was in the first year of middle school, one of my relatives told me about madhiy and told me that it is what comes out with desire. When I reached the third year of middle school, I took a lesson about madhiy and how to purify oneself from it. After that, every time I became junub I would wash my penis and do wudoo’ for prayer, because I did not know that it was maniy. When I learned the ruling nearly three years later, I was upset about the prayers that I offered (when I was junub). Do I have to repeat them because of my misunderstanding?.
Praise be to Allah.
      The difference between madhiy and maniy is well known. For 
      more information please see the answer to question no.
      2458. What the Muslim is 
      required to do is learn what he needs to know in order for his belief, 
      worship and dealings with others to be sound. 

    If a person offers many prayers without having purified 
    himself correctly, because he was unaware that purification was obligatory, 
    he does not have to repeat them, according to the correct opinion. 

    Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) 
    said:  

    Based on this, if a person did not purify himself as 
    prescribed in sharee’ah because the text had not reached him, such as if he 
    ate camel meat and did not do wudoo’, then the text reached him and he came 
    to know that it is obligatory to do wudoo’ after eating camel meat, or if he 
    prayed in a camel pen then the text reached him (which says that it is not 
    allowed to pray in camel pens), does he have to repeat the past prayers? 
    There are two opinions, both of which were narrated from Ahmad.  

    A similar case is if a man touches his penis and prays, then 
    he finds out that the one who touches his penis has to do wudoo’.  

    The correct view in all these cases is that he does not have 
    to repeat the prayers, because Allaah has forgiven things done by mistake or 
    out of forgetfulness, and because He says (interpretation of the meaning): 

    “And We never punish until We have sent a Messenger (to 
    give warning)”

    [al-Isra’ 17:15].  

    If a person has not heard of the command of the Messenger of 
    Allaah (peace and blessings of Allaah be upon him) concerning a particular 
    matter, then the obligation is not confirmed in his case. Hence when ‘Umar 
    and ‘Ammaar became junub and ‘Umar did not pray and ‘Ammaar prayed after 
    rolling in the dust, the Prophet (peace and blessings of Allaah be upon him) 
    did not tell either of them to repeat the prayer. Similarly he did not tell 
    Abu Dharr to repeat the prayers when he was junub and did not pray for 
    several days. And he did not tell those of his companions who had eaten 
    until they could distinguish the white thread from the black thread to make 
    up their fasts, and he did not tell those who had prayed facing Bayt 
    al-Maqdis (Jerusalem) before news of the abrogation reached them to make up 
    those prayers.  

    A similar case is that of the woman who was suffering from 
    istihaadah (non-menstrual vaginal bleeding) and had not prayed for some time 
    because she thought that she did not have to pray. There are two views as to 
    whether such a woman has to make up the prayers she missed. One view is that 
    she does not have to make up the prayers – as was narrated from Maalik and 
    others – because when the woman who was suffering from istihaadah said to 
    the Prophet (peace and blessings of Allaah be upon him): “I have been 
    bleeding heavily for some time and it prevents me from praying and fasting,” 
    he told her what she should do in the future, and he did not tell her to 
    make up the prayers of the past.  

     End quote from Majmoo‘ al-Fataawa, 21/101 

    See also the answer to question no.
    45648 

    But you should do a lot of naafil prayers, because the naafil 
    prayers will make up for any shortfall in the obligatory prayers. And you 
    should seek to learn beneficial knowledge, because the Prophet (blessings 
    and peace of Allah be upon him) said: “If Allaah intends good for a person, 
    He causes him to understand Islam.” Narrated by al-Bukhaari (71) and Muslim 
    (1037). 

    May Allah help us and you to do that which He loves and which 
    pleases Him. 

    And Allah knows best.