How is the zakat calculated for the orphan's money that the state withholds and invests for him until he reaches 21 years of age? Is zakat due on it annually or only upon receiving the money? And what is the ruling if the investment is in usurious banks due to state compulsion?

It is not permissible for the guardian of a minor to delay the payment of zakat (definition) until the minor reaches maturity. Rather, it must be paid every year at its legally appointed time. This is the opinion of the majority of scholars. If it has not been paid until he reaches maturity, then he must calculate the zakat for each previous year and pay it, without counting the zakat due in a previous year as part of the wealth in the following year. Similarly, he is not obligated to pay the profit from the zakat because it is a debt payable, and a debt is paid as it is.