Paying zakaah from a child’s wealth for past years
Question

                What is the ruling if a person did not pay zakaah for the past four years out of ignorance, and he has started to pay zakaah this year? What should he do about the past years?

 What is the ruling on shares that were held under trust? Should zakaah be paid on them when the shareowner reaches puberty and the age of accountability? Please note that the trustee has not paid zakaah on it for nearly 18 years.
Praise be to Allah.
    Firstly:

    Whoever has failed to pay zakaah for past years has to pay 
    it, whether he omitted to do so knowingly or out of ignorance. Shaykh Ibn 
    Baaz (may Allaah have mercy on him) was asked about someone who did not pay 
    zakaah for five years out of ignorance. He replied: 
You have to pay zakaah 
    for all the past years. Your being ignorant does not mean that it is waived 
    for you, because the obligation of zakaah is a well known part of the 
    religion which no Muslim has any excuse for not knowing, and the ruling is 
    not concealed from the Muslims. Zakaah is the third pillar of Islam. So you 
    must hasten to pay zakaah for all the past years, as well as repent to 
    Allaah for delaying it. May Allaah forgive us, you and all the Muslims. And 
    Allaah is the Source of strength. 

    End quote from Majmoo’ Fataawa al-Shaykh Ibn Baaz, 
    14/239 

    See also question no. 
    21715 

    An exception from that is zakaah on kinds of wealth where 
    there is some scholarly dispute as to whether zakaah is due on them, such as 
    jewellery that is used. Whoever does not pay zakaah on it out of ignorance, 
    or because he follows a Shaykh who says that zakaah is not due on it, then 
    he does not have to pay zakaah on it for the past, but he should pay zakaah 
    on it once he knows that it is subject to zakaah. 

    This was also stated in a fatwa by Shaykh Ibn Baaz (may 
    Allaah have mercy on him) who said: We also draw attention to the fact that 
    it is obligatory to pay zakaah once you know that it is required on 
    jewellery, but with regard to the past years before knowing that it was 
    required, you do not have to pay zakaah for that, because the rulings of 
    sharee’ah only become binding after one learns of them, and because there is 
    a scholarly difference of opinion on this issue. End quote from Fataawa 
    Islamiyyah, 2/84 

    Secondly: 

    The majority of scholars are of the view that zakaah is due 
    on the wealth of minors, and that being a minor does not mean that the 
    obligation to pay zakaah is waived. 

    Ibn Qudaamah (may Allaah have mercy on him) said: Zakaah must 
    be paid on the wealth of minors and the insane. That was narrated from 
    ‘Umar, ‘Ali, Ibn ‘Umar, ‘Aa’ishah, al-Hasan ibn ‘Ali and Jaabir (may Allaah 
    be pleased with them all). It was also the view of Maalik, al-Shaafa’i and 
    Ahmad. End quote from al-Mughni, 2/256. 

    This has already been discussed in the answer to question no
    75307. 

    Thirdly: 

    Not all shares are subject to zakaah. Some of them are 
    subject to zakaah and in other cases, zakaah is only due on the profits, if 
    they reach the nisaab and one year has passed. For more information on that, 
    please see the answer to question no 
    69912 . 

    If these shares are subject to zakaah, then he must pay 
    zakaah for the past years.  

    If zakaah is due on their profits only, then he must pay 
    zakaah on the profits that reach the nisaab, after one year has passed. 

    And Allaah knows best.