Henri M. said:
If you want to know the age or other regulations, just check the constitution. For an example, the Netherlands constitution says:
Article 33
The King shall not exercise the royal prerogative before attaining the age of eighteen.
Article 34
The guardianship of a King who is a minor shall be regulated by Act. The two Chambers of the States-General shall meet in joint assembly to consider and decide upon the matter.
On the moment there is no Act for the guardianship of a King because the Heir-Apparent is no minor. Before 1985 there was an Act to appoint a guardian over the King:
Article 1
1. During the period that the Heir-Apparent, born in the marriage of Us, Beatrix, with His Royal Highness Claus Georg Willem Otto Frederik Geert Prince of the Netherlands, Jonkheer van Amsberg, has become King by hereditary succession and can not exercise the royal prerogative because of not having attained the age of consent conform the Constitution, Our Consort aforementioned will act as Regent of the Kingdom,
and,
will Our Consort aforementioned have deceased before this period, Her Royal Highness Margriet Francisca Princess of the Netherlands, Princess of Orange-Nassau, Princess of Lippe-Biesterfeld will act as Regentess of the Kingdom.
Article 2
Might Our Consort beforementioned decease or abdicate the Regency or become unable to fulfill the Regency, Our Consort beforementioned will be succeeded by Her Royal Highness Margriet Francisca Princess of the Netherlands, Princess of Orange-Nassau, Princess of Lippe-Biesterfeld as Regentess of the Kingdom.