Basically zakat is a due on anybody who was owning the amount of nisab for a full lunar year there's no condition on his age -at least I don't know of any scholarly agreement on such-, but one would expect the parents to fulfill this duty for a child as long as it neither knows the ruling nor has access to money nor knows how to handle money.
As for your duty now after so many years:
First of all you must repent to Allah for your mistake which was hopefully out of ignorance of the rule, else it isn't just a sin, but kufr that you have committed.
Secondly you must start from now on to pay Zakat and you must pay for the missing years for this you must do a good guess. It might be helpful to check your bank account (history) and the gold and silver prices for the past years for both metals only the pure gold (21 karat) and pure silver must be considered.
See also fatwa islamqa #69940 where a statement of sheikh ibn 'Uthaymeen is mentioned:
Zakaah is an act of worship of Allaah, and it a right owed to the people who are entitled to zakaah. If a person withholds it then he is transgressing two rights, the right of Allaah, may He be exalted, and the right of the people who are entitled to zakaah. If he repents after five years – as it says in the question – then the right of Allaah is waived, because Allaah says (interpretation of the meaning):”
“And He it is Who accepts repentance from His slaves, and forgives sins, and He knows what you do”
(al-Shoora 42:25)
But there remains the second right – that of those who are entitled to zakaah such as the poor and others, so he must give the zakaah to them. He may attain the reward for zakaah if his repentance is sincere, because the grace of Allaah is immense.
With regard to working out the zakaah, try to work out the amount of zakaah as best you can, and Allaah does not burden any soul beyond its scope. The zakaah of ten thousand – for example – is two hundred and fifty per year. If the amount of zakaah is two hundred and fifty, then you should pay two hundred and fifty for each of the past years, unless there was more than ten thousand in one year, in which case you should pray proportionately more; if it decreased in some years then the zakaah decreases accordingly. End quote.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 18/question no. 214
