Confirmation can be conferred only on those who have already been baptised and have not yet been confirmed. As St. Thomas says:
Confirmation is to baptism what growth is to generation. Now it is clear that a man cannot advance to a perfect age unless he has first been born; in like manner, unless he has first been baptised he cannot receive the Sacrament of Confirmation (Summa Theologiæ III.72.6).
They should also be in the state of grace; for the Holy Ghost is not given for the purpose of taking away sin but of conferring additional grace. This condition, however, refers only to lawful reception; the sacrament is validly received even by those in mortal sin. In the early ages of the Church, confirmation was part of the rite of initiation, and consequently was administered immediately after baptism. When, however, baptism came to be conferred by simple priests, the two ceremonies were separated in the Western Church. Further, when infant baptism became customary, confirmation was not administered until the child had attained the use of reason. This is the present practice, though there is considerable latitude as to the precise age. The Catechism of the Council of Trent says that the sacrament can be administered to all persons after baptism, but that this is not expedient before the use of reason; and adds that it is most fitting that the sacrament be deferred until the child is seven years old, “for Confirmation has not been instituted as necessary for salvation, but that by virtue thereof we might be found well armed and prepared when called upon to fight for the faith of Christ, and for this kind of conflict no one will consider children, who are still without the use of reason, to be qualified.” (Pt. II, ch. iii, 18.)
Such, in fact, is the general usage in the Western Church. Under certain circumstances, however, as, for instance, danger of death, or when the opportunity of receiving the sacrament is but rarely offered, even younger children may be confirmed. In the Greek Church and in Spain, infants are now, as in earlier times, confirmed immediately after baptism. Leo XIII, writing 22 June, 1897, to the Bishop of Marseilles, commends most heartily the practice of confirming children before their first communion as being more in accord with the ancient usage of the Church.
Although Confirmation is sometimes called the “sacrament of Christian maturity,” we must not confuse adult faith with the adult age of natural growth, nor forget that the baptismal grace is a grace of free, unmerited election and does not need “ratification” to become effective. St. Thomas Aquinas reminds us of this: Age of body does not determine age of soul. Even in childhood man can attain spiritual maturity: as the book of Wisdom says: For old age is not honoured for length of time, or measured by number of years. Many children, through the strength of the Holy Spirit they have received, have bravely fought for Christ even to the shedding of their blood. – source
The “soldier of Christ” imagery, remains valid but is downplayed if seen as part of the once common idea of Confirmation as a “sacrament of maturity”,was used as far back as 350, by St Cyril of Jerusalem. In this connection, the touch on the cheek that the bishop gave while saying “Pax tecum” (Peace be with you) to the person he had just confirmed was interpreted in the Roman Pontifical as a slap, a reminder to be brave in spreading and defending the faith: “Deinde leviter eum in maxilla caedit, dicens: Pax tecum” (Then he strikes him lightly on the cheek, saying: Peace be with you) (cf. the knightly custom of the accolade). When, in application of the Second Vatican Council’s Constitution on the Sacred Liturgy, the Confirmation rite was revised in 1971, mention of this gesture was omitted. However, the French and Italian translations, indicating that the bishop should accompany the words “Peace be with you” with “a friendly gesture” (French text) or “the sign of peace” (Italian text), explicitly allow a gesture such as the touch on the cheek, to which they restore its original meaning.
This is in accord with the Introduction to the Rite of Confirmation, 17, which indicates that the episcopal conference may decide “to introduce a different manner for the minister to give the sign of peace after the anointing, either to each individual or to all the newly confirmed together – source