Objection 1. It seems that this sacrament should not be given to all. For this sacrament is given in order to confer a certain excellence, as stated above (11, ad 2). But all are not suited for that which belongs to excellence. Therefore this sacrament should not be given to all.
Objection 2. Further, by this sacrament man advances spiritually to perfect age. But perfect age is inconsistent with childhood. Therefore at least it should not be given to children.
Objection 3. Further, as Pope Melchiades says (Ep. ad Episc. Hispan.) "after Baptism we are strengthened for the combat." But women are incompetent to combat, by reason of the frailty of their sex. Therefore neither should women receive this sacrament.
Objection 4. Further, Pope Melchiades says (Ep. ad Episc. Hispan.): "Although the benefit of Regeneration suffices for those who are on the point of death, yet the graces of Confirmation are necessary for those who are to conquer. Confirmation arms and strengthens those to whom the struggles and combats of this world are reserved. And he who comes to die, having kept unsullied the innocence he acquired in Baptism, is confirmed by death; for after death he can sin no more." Therefore this sacrament should not be given to those who are on the point of death: and so it should not be given to all.
On the contrary, It is written (Acts 2:2) that the Holy Ghost in coming, "filled the whole house," whereby the Church is signified; and afterwards it is added that "they were all filled with the Holy Ghost." But this sacrament is given that we may receive that fulness. Therefore it should be given to all who belong to the Church.
I answer that, As stated above (Article 1), man is spiritually advanced by this sacrament to perfect age. Now the intention of nature is that everyone born corporally, should come to perfect age: yet this is sometimes hindered by reason of the corruptibility of the body, which is forestalled by death. But much more is it God's intention to bring all things to perfection, since nature shares in this intention inasmuch as it reflects Him: hence it is written (Deuteronomy 32:4): "The works of God are perfect." Now the soul, to which spiritual birth and perfect spiritual age belong, is immortal; and just as it can in old age attain to spiritual birth, so can it attain to perfect (spiritual) age in youth or childhood; because the various ages of the body do not affect the soul. Therefore this sacrament should be given to all.
Reply to Objection 1. This sacrament is given in order to confer a certain excellence, not indeed, like the sacrament of order, of one man over another, but of man in regard to himself: thus the same man, when arrived at maturity, excels himself as he was when a boy.
Reply to Objection 2. As stated above, the age of the body does not affect the soul. Consequently even in childhood man can attain to the perfection of spiritual age, of which it is written (Wisdom 4:8): "Venerable old age is not that of long time, nor counted by the number of years." And hence it is that many children, by reason of the strength of the Holy Ghost which they had received, fought bravely for Christ even to the shedding of their blood.
Reply to Objection 3. As Chrysostom says (Hom. i De Machab.), "in earthly contests fitness of age, physique and rank are required; and consequently slaves, women, old men, and boys are debarred from taking part therein. But in the heavenly combats, the Stadium is open equally to all, to every age, and to either sex." Again, he says (Hom. de Militia Spirit.): "In God's eyes even women fight, for many a woman has waged the spiritual warfare with the courage of a man. For some have rivaled men in the courage with which they have suffered martyrdom; and some indeed have shown themselves stronger than men." Therefore this sacrament should be given to women.
Reply to Objection 4. As we have already observed, the soul, to which spiritual age belongs, is immortal. Wherefore this sacrament should be given to those on the point of death, that they may be seen to be perfect at the resurrection, according to Ephesians 4:13: "Until we all meet into the unity of faith... unto the measure of the age of the fulness of Christ." And hence Hugh of St. Victor says (De Sacram. ii), "It would be altogether hazardous, if anyone happened to go forth from this life without being confirmed": not that such a one would be lost, except perhaps through contempt; but that this would be detrimental to his perfection. And therefore even children dying after Confirmation obtain greater glory, just as here below they receive more grace. The passage quoted is to be taken in the sense that, with regard to the dangers of the present combat, those who are on the point of death do not need this sacrament.
Source: Summa Theologica (New Advent)