Is zakat obligatory on orphans' money, collected in the bank and deposited by their guardian, given that each orphan's share does not reach the nisab if calculated individually, and how is zakat to be paid for past years if it was obligatory?

As long as each orphan's share of the money does not individually reach the nisab, there is no zakat due on them unless one of them possesses other funds that, when added to it, complete the nisab. And for whoever's share reaches the nisab (equivalent to 85 grams of gold or 595 grams of silver), the guardian must annually pay one-quarter of one-tenth as zakat on their behalf.