Those who are required to pay Zakat are equally required to pay missed Zakat for those years which they qualified to pay Zakat but failed to do so. At the time of missing a Zakat payment, those required to pay missed Zakat must be:
• Adult (have reached puberty)
• Muslim
• Sane
• In complete ownership of the Nisab (Add link for info)

Please note, the three schools of thought other than the Hanafi school state that Zakat should be paid on qualifying wealth owned by the insane and children.

And Allah knows best!

Mufti Faraz Adam