The Prophet's Fasting System

The fast of Ramadan, kalat, is making up missed days that had a valid excuse. You may fast on nafila in any day, except the first of shawal, which is Eid. The only days that's ever forbidden to fast are the days of Eid. When and when you are mature, physically mature means you pass puberty. Your puberty is marked by the moment a boy releases semen. He's now a man in the law.