How can one make up the days of Ramadan that were not fasted since puberty, given that they were remembered years later and the reason for not fasting them was ignorance of puberty?

What you saw when you were eight years old is not menstruation, for the minimum age for menstruation is nine years. And what you saw before Ramadan is not considered menstruation by the majority of scholars because its duration did not reach a day and a night, and the minimum menstruation according to them is a day and a night.

As for the view of the Malikis, there is no minimum limit for menstruation, and what you saw before Ramadan is considered menstruation, and you are required to make up those days.

An ordinary person follows whoever they trust among the people of knowledge. And if you choose to make up those days as a precaution, there is no expiation (fidyah) upon you.