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.
Source: Catechism of the Catholic Church (Vatican.va)