12 In order that the spiritual training rest upon a more solid basis and that the students embrace their vocation with a fully deliberate choice, it will be the prerogative of the bishops to establish a fitting period of time for a more intense introduction to the spiritual life. It will also be their charge to determine the opportuneness of providing for a certain interruption in the studies or of establishing a suitable introduction to pastoral work, in order that they may more satisfactorily test the fitness of candidates for the priesthood. In accordance with the conditions of individual regions it will also be the bishops' responsibility to make a decision about extending the age beyond that demanded at present by common law for the reception of sacred orders, and of deliberating whether it be opportune to rule that students, at the end of their course in theology, exercise the order of deacon for a fitting period of time before being promoted to the priesthood.
Source: Optatam Totius (Vatican.va)