Objection 1. It would seem that children ought not to be received in religion. Because it is said (Extra, De Regular. et Transeunt. ad Relig., cap. Nullus): "No one should be tonsured unless he be of legal age and willing." But children, seemingly, are not of legal age; nor have they a will of their own, not having perfect use of reason. Therefore it seems that they ought not to be received in religion.
Objection 2. Further, the state of religion would seem to be a state of repentance; wherefore religion is derived [Cf. 81, 1 from "religare" [to bind] or from "re-eligere" [to choose again], as Augustine says (De Civ. Dei x, 3 [Cf. De Vera Relig. lv]). But repentance does not become children. Therefore it seems that they should not enter religion.
Objection 3. Further, the obligation of a vow is like that of an oath. But children under the age of fourteen ought not to be bound by oath (Decret. XXII, qu. v, cap. Pueri and cap. Honestum.). Therefore it would seem that neither should they be bound by vow.
Objection 4. Further, it is seemingly unlawful to bind a person to an obligation that can be justly canceled. Now if any persons of unripe age bind themselves to religion, they can be withdrawn by their parents or guardians. For it is written in the Decretals (XX, qu. ii, can. Puella) that "if a maid under twelve years of age shall take the sacred veil of her own accord, her parents or guardians, if they choose, can at once declare the deed null and void." It is therefore unlawful for children, especially of unripe age, to be admitted or bound to religion.
On the contrary, our Lord said (Matthew 19:14): "Suffer the little children, and forbid them not to come to Me." Expounding these words Origen says (Tract. vii in Matth.) that "the disciples of Jesus before they have been taught the conditions of righteousness [Cf. Matthew 19:16-30, rebuke those who offer children and babes to Christ: but our Lord urges His disciples to stoop to the service of children. We must therefore take note of this, lest deeming ourselves to excel in wisdom we despise the Church's little ones, as though we were great, and forbid the children to come to Jesus."
I answer that, As stated above (2, ad 1), the religious vow is twofold. One is the simple vow consisting in a mere promise made to God, and proceeding from the interior deliberation of the mind. Such a vow derives its efficacy from the divine law. Nevertheless it may encounter a twofold obstacle. First, through lack of deliberation, as in the case of the insane, whose vows are not binding [Extra, De Regular. et Transeunt. ad Relig., cap. Sicut tenor]. The same applies to children who have not reached the required use of reason, so as to be capable of guile, which use boys attain, as a rule, at about the age of fourteen, and girls at the age of twelve, this being what is called "the age of puberty," although in some it comes earlier and in others it is delayed, according to the various dispositions of nature. Secondly, the efficacy of a simple vow encounters an obstacle, if the person who makes a vow to God is not his own master; for instance, if a slave, though having the use of reason, vows to enter religion, or even is ordained, without the knowledge of his master: for his master can annul this, as stated in the Decretals (Dist. LIV, cap. Si servus). And since boys and girls under the age of puberty are naturally in their father's power as regards the disposal of their manner of life, their father may either cancel or approve their vow, if it please him to do so, as it is expressly said with regard to a woman (Numbers 30:4).
Accordingly if before reaching the age of puberty a child makes a simple vow, not yet having full use of reason, he is not bound in virtue of the vow; but if he has the use of reason before reaching the age of puberty, he is bound, so far as he is concerned, by his vow; yet this obligation may be removed by his father's authority, under whose control he still remains, because the ordinance of the law whereby one man is subject to another considers what happens in the majority of cases. If, however, the child has passed the age of puberty, his vow cannot be annulled by the authority of his parents; though if he has not the full use of reason, he would not be bound in the sight of God.
The other is the solemn vow which makes a man a monk or a religious. Such a vow is subject to the ordinance of the Church, on account of the solemnity attached to it. And since the Church considers what happens in the majority of cases, a profession made before the age of puberty, however much the person who makes profession may have the use of reason, or be capable of guile, does not take effect so as to make him a religious (Extra, De Regular., etc. cap. Significatum est.).
Nevertheless, although they cannot be professed before the age of puberty, they can, with the consent of their parents, be received into religion to be educated there: thus it is related of John the Baptist (Luke 1:80) that "the child grew and was strengthened in spirit, and was in the deserts." Hence, as Gregory states (Dial. ii, 3), "the Roman nobles began to give their sons to the blessed Benedict to be nurtured for Almighty God"; and this is most fitting, according to Lamentations 3:27, "It is good for a man when he has borne the yoke from his youth." It is for this reason that by common custom children are made to apply themselves to those duties or arts with which they are to pass their lives.
Reply to Objection 1. The legal age for receiving the tonsure and taking the solemn vow of religion is the age of puberty, when a man is able to make use of his own will; but before the age of puberty it is possible to have reached the lawful age to receive the tonsure and be educated in a religious house.
Reply to Objection 2. The religious state is chiefly directed to the attachment of perfection, as stated above (186, 1, ad 4); and accordingly it is becoming to children, who are easily drawn to it. But as a consequence it is called a state of repentance, inasmuch as occasions of sin are removed by religious observances, as stated above (186, 1, ad 4).
Reply to Objection 3. Even as children are not bound to take oaths (as the canon states), so are they not bound to take vows. If, however, they bind themselves by vow or oath to do something, they are bound in God's sight, if they have the use of reason, but they are not bound in the sight of the Church before reaching the age of fourteen.
Reply to Objection 4. A woman who has not reached the age of puberty is not rebuked (Numbers 30:4) for taking a vow without her parents' consent: but the vow can be made void by her parents. Hence it is evident that she does not sin in vowing. But we are given to understand that she binds herself by vow, so far as she may, without prejudice to her parents' authority.
Source: Summa Theologica (New Advent)