What is the ruling regarding past years for which zakat was not paid due to ignorance, and what is the ruling regarding zakat on shares that were under guardianship and for which zakat was not paid for 18 years?

If someone has not paid zakat for years, they are obliged to pay it for all the past years, even if they were ignorant [of the obligation], because the فرض (obligatory act) of zakat is known by necessity to be part of the religion. This is unless the zakat pertains to wealth whose zakat obligation is disputed, such as used jewelry; in this case, zakat is not obligatory for the past period before one became aware of its obligation. Zakat is obligatory on the wealth of a child, and their young age does not negate its obligation. As for shares (stocks), some are subject to zakat on the principal amount, while others are subject to zakat only on their profits if they reach the nisab (minimum threshold) and a hawl (a lunar year) has passed over them. Their zakat for past years must be paid according to their type.