Council of Trent: Session 23
Chapter 11
The interstices, and certain other regulations, to be observed in receiving minor orders.
The minor orders shall not be given but to such as understand the Latin language at least, observing the appointed interstices of time, unless the bishop shall think it more expedient to act otherwise; that so they may be the more accurately taught how great is the obligation of this their state of life; and may exercise themselves in each office, agreeably to the appointment of the bishop; and this in the church to which they shall be assigned, unless they happen to be absent on account of their studies; and may thus ascend step by step: that so with their increasing age they may grow in worthiness of life and in learning; of which they will give proof especially by the example of their good conduct, by their assiduous service in the church, their greater reverence towards priests and the superior orders, and by a more frequent communion than heretofore of the Body of Christ. And whereas from these orders is the entrance unto higher orders and to the most sacred mysteries, no one shall be admitted thereunto, whom the promise of knowledge does not point out as worthy of the greater orders. And such shall not be promoted to sacred orders till a year after the reception of the last degree of minor orders; unless necessity, or the utility of the church, in the bishop's judgment, shall require otherwise.
Chapter 12
Age required for the major orders; the deserving only to be admitted.
No one shall for the future be promoted to the order of subdeaconship before the twenty-second year of age; to that of deaconship before his twenty-third year; to that of priesthood before his twenty-fifth year. Nevertheless, bishops are to know, that not all who have attained to that age must needs be admitted to the aforesaid orders, but those only who are worthy, and whose commendable life is an old age. Regulars likewise shall not be ordained under the above age, nor without a diligent examination by the bishop; all privileges whatsoever in this regard being completely set aside.
Chapter 13
On the conditions required in the Ordination of a Subdeacon and Deacon: on no one shall two sacred Orders be conferred on the same day.
Such as have a good testimonial, and have been already tried in minor orders, and are instructed in letters, and in those things which belong to the exercise of their orders, shall be ordained subdeacons and deacons. They shall have a hope, with God's help, to be able to live continently; they shall serve in the churches to which they may be assigned; and are to know that it is very highly becoming that, after ministering at the altar, they should receive the sacred communion, at least on the Lord's days and solemnities. Those who have been promoted to the sacred order of the subdeaconship shall not, until they have remained therein during at least a year, be permitted to ascend to a higher degree, unless the bishop shall judge otherwise. Two sacred orders shall not be conferred on the same day, even upon Regulars; any privileges and indults whatsoever, to whomsoever granted, to the contrary notwithstanding.