What should I do if I reached puberty at a young age and fasted many Ramadans, but I do not remember whether I made up for any of them that I missed?

Making up for missed days of Ramadan is obligatory, based on the Almighty's saying: "A [like] number of other days" (Quran 2:184), and Aisha's statement: "We were commanded to make up for the fast, but not for the prayer." One must ascertain the number of missed days, and if a specific number cannot be determined, one should fast the number of days in doubt until one's obligation is fulfilled. A minor expiation (kaffarah sughra) is also obligatory, which involves feeding one poor person for each day whose make-up was delayed until the next Ramadan without a valid excuse.