Minority reign

The term minority reign or royal minority refers to the period of a sovereign's rule when he or she is legally a minor. Minority reigns are of their nature times when politicians and advisors can be especially competitive. Some scholars claim that, in Britain, primogeniture, the growth of conciliar government, and the emergence of the Parliament as a representative and administrative force all occurred within the context of the minority reigns.
Minority reigns also characterized a period in the Roman empire (367-455), notably the years that preceded the reign of Valentinian III, who also became emperor when he was six years old. The succession of child-turned-adult emperors led to the so-called infantilization of the imperial office, which had taken hold during the long reign of Honorius, Valentinian's predecessor. Here, the imperial office operated within a severely curtailed system compared to its authority a century prior. 
Authority.
Commonly, a regent is appointed if a sovereign is a minor. There are cases when no regent is appointed but these did not mean that the monarch held authority. For example, during the minority reign of Theodosius II, power was wielded by Anthemius before his sister Pulcheria was appointed the regent. In many instances, the advent of a royal minority led to fierce competition for any regency office, and in England only one actual regent was ever appointed: In October 1216 William Marshal, 1st Earl of Pembroke became regent for the nine-year-old Henry III on the death of King John. Subsequent royal minorities before 1811 were dealt with by the appointment of officers who held the less provocative title "Lords Justices of the Realm", "Lord Protector" or "Protector and Defender" (after 1422), and sometimes "Guardian of the Realm". In all instances they were intended to be assisted by a collective council or body of officials, although the brief Protectorate of Richard, duke of Gloucester from April to June 1483 did not allow for the naming of an official council.
Sovereigns who have ruled as minors include: