If someone missed 20 years of fasting due to ignorance, what should one do?
If he knew that fasting was obligatory upon him yet he chose to not fast, he must repent to Allah Exalted and Glorified, and must make up the days he missed, as well as offer a kaffarah of feeding sixty poor people for each missed day or fast sixty consecutive days for each missed day.
For more details see this, please: https://imam-us.org/reminder-about-qada-make-up-fasts-fidyah-kaffarah-1441/