Although, in the West, Confirmation is usually received as a teenager, several years after making First Communion, the Catholic Church considers it the second of the three Sacraments of Initiation (Baptism being the first and Communion the third). Confirmation is regarded as the perfection of Baptism, because, as the introduction to the Rite of Confirmation states:
by the sacrament of Confirmation, [the baptized] are more perfectly bound to the Church and are enriched with a special strength of the Holy Spirit. Hence they are, as true witnesses of Christ, more strictly obliged to spread and defend the faith by word and deed.
Even in the West, priests can be authorized by their bishops to perform confirmations, and adult converts are routinely baptized and confirmed by priests. All those who have been baptized are eligible to be confirmed, and, while the Western Church suggests receiving the sacrament after reaching the "age of reason" (around seven years old), it can be received at any time. (A child in danger of death should receive Confirmation.)
A confirmand must be in a state of grace. If the sacrament is not received immediately after Baptism, the confirmand should participate in the Sacrament of Confession before Confirmation.
Paragraphs 1294 and 1295 By Confirmation Christians, that is, those who are anointed, share more completely in the mission of Jesus Christ and the fullness of the Holy Spirit with which he is filled, so that their lives may give off "the aroma of Christ." By this anointing the confirmand receives the "mark," the seal of the Holy Spirit. A seal is a symbol of a person, a sign of personal authority, or ownership of an object. Hence soldiers were marked with their leader's seal and slaves with their master's. A seal authenticates a juridical act or document and occasionally makes it secret.
Paragraph 1299 In the Roman Rite the bishop extends his hands over the whole group of the confirmands. Since the time of the apostles this gesture has signified the gift of the Spirit. The bishop invokes the outpouring of the Spirit in these words:
All-powerful God, Father of our Lord Jesus Christ,
by water and the Holy Spirit
you freed your sons and daughters from sin
and gave them new life.
Send your Holy Spirit upon them
to be their helper and guide.
Give them the spirit of wisdom and understanding,
the spirit of right judgment and courage,
the spirit of knowledge and reverence.
Fill them with the spirit of wonder and awe in your presence.
We ask this through Christ our Lord.
Paragraph 1303b (quoting St. Ambrose) Guard what you have received. God the Father has marked you with his sign; Christ the Lord has confirmed you and has placed his pledge, the Spirit, in your hearts.