Through the Sacrament of Confirmation, also called Chrismation (in the East), God perfects the Holy Spirit’s graces given to the soul in Baptism. The graces the soul received at Baptism are strengthened, and the confirmand is enabled all the more to be a Soldier of Christ in the world. In essence, Confirmation gives confirmandi more graces to become great saints, which is every single person’s only goal in life – to do God’s will at every moment of our lives through death.
The Seven Gifts of the Holy Spirit were communicated to Christ for His earthly mission (even though He possessed them in all fullness beforehand) and thus gives us an example. At the beginning of Christ’s ministry, He let St. John the Baptist baptize Him, and the Spirit descended “upon him like a dove” (Mk 1:10). This was done to fulfill the Isaiah’s Old Testament prophecy in reference to the Messiah: “The Spirit of the Lord shall rest upon Him, the spirit of wisdom and understanding, the spirit of counsel and might, the spirit of knowledge and fear of the Lord. And his delight shall be in the fear of the Lord” (Is 11:2-3).
So we receive the Holy Spirit’s gifts in an exalted manner at our Confirmations, in order to strengthen us for the constant fight against evil in the world.
The Sacrament of Confirmation is a sacrament of initiation in the sense that it (along with Baptism and the Most Holy Eucharist) initiates one into full participation in the Christian life. However, it does not initiate one into “spiritual adulthood,” or making one “an adult in the Church.” It is not a rite of passage in any sense.
The matter of the sacrament is Chrism Oil, which the bishop consecrates on Maundy (Holy) Thursday. The form are the words: “N., be sealed with the gift of the Holy Spirit,” while the minister anoints the confirmand’s head with oil, and lays his hands on his head. The ordinary minister of Confirmation in the Western Church is the bishop, while in the Eastern Church it is any priest. In the Western Church, a pastor of a parish can receive permission to confirm, but this is an exception.
In the Eastern Rites of the Catholic Church, infants are baptized, chrismated (confirmed), and receive the Most Holy Eucharist all at once. The Sacrament of Confirmation perfects the baptismal graces in the confirmand’s soul. Thus, there is no barrier to infants receiving the sacrament. As infants do not need to be conscious of being baptized in order to receive the sacrament, they do not need to be conscious of Confirmation in order to receive its grace.
So why are Catholics in the West usually confirmed around young adult age? It has more to do with logistical situations than for theological reasons. As the Church grew, it got harder for bishops to travel from place to place to minister the sacrament. So Catholics had to wait much longer to be Confirmed. This started to happen en masse around the 13th century. For several hundred years after that, Catholics generally received Confirmation before Holy Communion, though this changed around the turn of the 20th century as Pope St. Pius X lowered the age for First Holy Communion.