Young kings of England
Since the reign of the first kings of England the question of who should rule next has been hard to answer. Examples of this are when Edward the Elder died leaving two sons Aelfward and Athelstan, but Aelfward died 28 days after Edward leaving the throne to Athelstan. But when a king picked his successor in the 1100s it was known as the young king. The next king, the prince of Wales usually got to reign as the young king of England before his ascension but both of the young/junior/titular kings never ascended to the throne.