Objection 1. It would seem that boys and those who lack the use of reason cannot receive Orders. For, as stated in the text (Sent. iv, D, 25), the sacred canons have appointed a certain fixed age in those who receive Orders. But this would not be if boys could receive the sacrament of Orders. Therefore, etc.
Objection 2. Further, the sacrament of Orders ranks above the sacrament of matrimony. Now children and those who lack the use of reason cannot contract matrimony. Neither therefore can they receive Orders.
Objection 3. Further, act and power are in the same subject, according to the Philosopher (De Somn. et Vigil. i). Now the act of Orders requires the use of reason. Therefore the power of Orders does also.
On the contrary, one who is raised to Orders before the age of discretion is sometimes allowed to exercise them without being reordained, as appears from Extra., De Cler. per salt. prom. But this would not be the case if he had not received Orders. Therefore a boy can receive Orders.
Further, boys can receive other sacraments in which a character is imprinted, namely Baptism and Confirmation. Therefore in like manner they can receive Orders.
I answer that, Boyhood and other defects which remove the use of reason occasion an impediment to act. Wherefore the like are unfit to receive all those sacraments which require an act on the part of the recipient of the sacrament, such as Penance, Matrimony, and so forth. But since infused powers like natural powers precede acts--although acquired powers follow acts--and the removal of that which comes after does not entail the removal of what comes first, it follows that children and those who lack the use of reason can receive all the sacraments in which an act on the part of the recipient is not required for the validity of the sacrament, but some spiritual power is conferred from above; with this difference, however, that in the minor ordersthe age of discretion is required out of respect for the dignity of the sacrament, but not for its lawfulness, nor for its validity. Hence some can without sin be raised to the minor ordersbefore the years of discretion, if there be an urgent reason for it and hope of their proficiency. and they are validly ordained; for although at the time they are not qualified for the offices entrusted to them, they will become qualified by being habituated thereto. For the higher Orders, however, the use of reason is required both out of respect for, and for the lawfulness of the sacrament, not only on account of the vow of continency annexed thereto, but also because the handling of the sacraments is entrusted to them [See Acts of the Council of Trent: De Reform., Sess. xxii, cap. 4,11,12. But for the episcopate whereby a man receives power also over the mystical body, the act of accepting the pastoral care of souls is required; wherefore the use of reason is necessary for the validity of episcopal consecration. Some, however, maintain that the use of reason is necessary for the validity of the sacrament in all the Orders. but this statement is not confirmed either by authority or by reason.
Reply to Objection 1. As stated in the Article, not all that is necessary for the lawfulness of a sacrament is required for its validity.
Reply to Objection 2. The cause of matrimony is consent, which cannot be without the use of reason. Whereas in the reception of Orders no act is required on the part of the recipients since no act on their part is expressed in their consecration. Hence there is no comparison.
Reply to Objection 3. Act and power are in the same subject; yet sometimes a power, such as the free-will, precedes its act; and thus it is in the case in point.
Source: Summa Theologica (New Advent)