Shaykh Ibn Baaz (may Allaah have mercy on him) was asked about not paying zakaah for five years out of ignorance.
He replied: You have to pay zakaah for all the past years, and it is not waived due to your ignorance, because the obligation of zakaah is something that no Muslim has any excuse for not knowing, and the ruling is well known to all Muslims. Zakaah is the third pillar of Islam. What you must do is hasten to pay zakaah for all the past years, and repent to Allaah, may He be glorified and exalted, for delaying it. May Allaah forgive us and you and all the Muslims. And Allaah is the Source of strength.
End quote from Majmoo’ Fataawa al-Shaykh Ibn Baaz (14/239).