Baptism, the Eucharist, and the sacrament of Confirmation together constitute the ‘sacraments of Christian initiation’, whose unity must be safeguarded. It must be explained to the faithful that the reception of the sacrament of Confirmation is necessary for the completion of baptismal grace. For ‘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’.
Catechism of the Catholic Church, n. 1285 (cf. Lumen Gentium 11)
The Sacrament of Confirmation imparts the special Grace of the Holy Spirit to be witnesses to Christ in what we say and do each day of our lives: we are anointed on the forehead with the Oil of Chrism, with the words ‘Be sealed with the gift of the Holy Spirit’. Being sealed, as the letter to the Ephesians 1:13 reminds us, means that we are marked as belonging to Christ.
In the Catholic tradition, it is usual to receive the sacrament of Confirmation once a young person has reached an age where they have begun to make choices for themselves. The Diocese of Leeds invites children from Year 5 to begin preparation, receiving the sacrament in Year 6; whilst adults being received into the Church are prepared through the Rite of Christian Initiation of Adults (RCIA) and receive this sacrament, together with Baptism and Eucharist, usually at the Easter Vigil.
The following books may be of interest if you would like to learn more: