"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 8:14-17

1316 Confirmation perfects Baptismal grace; it is the sacrament which gives the Holy Spirit in order to root us more deeply in the divine filiation, incorporate us more firmly into Christ, strengthen our bond with the Church, associate us more closely with her mission, and help us bear witness to the Christian faith in words accompanied by deeds.

1317 Confirmation, like Baptism, imprints a spiritual mark or indelible character on the Christian's soul; for this reason one can receive this sacrament only once in one's life.

1308 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 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 honored 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.