Must I make up or only offer expiation for the days I did not fast during Ramadan when I was fourteen, believing that I was not yet religiously obligated, knowing that I later learned the age of religious obligation is fifteen?

If the signs of puberty were not present when fasting was not observed, then no make-up fasting (qada') is required. However, if one of the signs of puberty was present, then the days that were not fasted must be made up, along with seeking forgiveness (istighfar), and no expiation (kaffarah) is due, according to the preponderant opinion.