Must I make up for the fasting only, or is there something else I must do, given that I broke my fast for three days in Ramadan when I was 13 years old and had not yet reached puberty, and I just remembered this now that I am 16 years old?

A child is not obligated to fulfill religious duties until after reaching puberty. Therefore, he is not required to make up the days he missed fasting before puberty. His fasting before puberty is considered a voluntary act for which he will be rewarded, but it is not considered making up an obligatory duty. A person is considered to have reached puberty if he reaches 15 years of age, or ejaculates semen, or grows coarse hair around the pubic area.