What is the ruling on one who was unaware of the signs of puberty and believed that fasting became obligatory at 13 years of age, whereas he reached puberty at 12, and thus did not fast Ramadan at that time? Is he required to make up for that month?

If a person shows one of the signs of puberty at the age of twelve, then they were obligated to fast Ramadan in that year. However, if they did not fast out of ignorance of its obligation, the sin is lifted from them, but they must make up the missed fasts, because not knowing about the obligation does not negate the obligation itself, but rather lifts the sin.