“the reception of the sacrament of Confirmation is necessary for the completion of baptismal grace.” - Catechism of the Catholic Church, Part Two, Section Two, Chapter 1, Article 2, Paragraph 1285.
Now when the apostles in Jerusalem heard that Samaria had accepted the word of God, they sent them Peter and john, who went down and prayed for them, that they might receive the holy Spirit, for it had not yet fallen upon any of them; they had only been baptized in the name of the Lord Jesus. Then they laid hands on them and they received the Holy Spirit - Acts 8: 14-17In the Roman Catholic Church, the Sacrament of Baptism is given to those of any age, but the Sacrament of Confirmation is given only to those who can consent to the battle. It is wise to armor all, since no one is exempt from the battlefield because Satan is no respecter of age, but it would be unwise to equip all with weapons until they were both willing and able to use them. The Church, in her wisdom, requires that the child have reached the age of wisdom and has sufficient spiritual maturity to understand the stakes involved.
Many children, through the strength of the Holy Spirit they have received, have bravely fought for Christ even to the shedding of their blood. - St. Thomas Aquinas, STh III,72,8, ad 2; Cf. Wis 4:8.What Baptism begins with water, confirmation finishes with oil. Thus, the initiate is washed clean before he is anointed to take his place among the chosen of God. This anointing calls to mind the anointing of the prophets of old as well as the anointing of the kings of Israel, since Christ to whom the initiate has been joined is Priest, Prophet, and King. It signifies their new role in the Church and the responsibilities which that role requires them to accept.
Confirmation is ordinarily administered by the bishop. The bishop is a successor of the original apostles, and has received the fullness of the sacrament of Holy Orders. However, when the need arises or the possibility of the Bishop’s administration is impractical, the priest may administer the sacrament.