This is a tough question to answer - nearly impossible, in fact - unless a consensus is reached as to what exactly the Sacrament of Confirmation is. There seem to be many versions of the “purpose” (for lack of a better term) for even recieving this Sacrament, each one of which will point to a different appropriate age.
In the Catholic East, the Holy Mystery of Chrismation (the Sacrament of Confirmation in the Latin Rite) is conferred immediately after Baptism, with the thought being that this Mystery brings special graces of the Holy Spirit to the recipient, thus strengthening the union with Christ and His Church that was just created through Baptism. Numerous Scriptural references can be found indicating that the Apostles would immediately “lay hands” on newly baptized Christians in order to bring down the Grace of the Holy Spirit unto them as soon as possible, thus strengthening that union. In this scenario, infant Chrismation is proper and appropriate.
In the West, it seems as though the Sacrament of Confirmation has evolved into a sort of “rite of passage.” The same Gifts of the Holy Spirit are imparted to the recipient, but the thrust of the Sacrament, for many Westerners, seems to be the recipient’s acceptance of his role as a “Soldier of the Church.” This acceptance of that role would certainly indicate the need for an accepted age of reason to be reached before receiving the Sacrament.
Others in the West see it as an act of becoming an “adult” member of the Church, which may call for yet a different, somewhat older, age to be the appropriate one.
Still others in the West would like to see the Sacrament revert to its Scriptural origins as a “rite of initiation” into the Catholic Church, which would place the appropriate age for reception at the same point it currently is in the Churches of the East.
Perhaps there needs to be universal acceptance and understanding of what the Sacrament of Confirmation is and does before one can answer when it should be received.
a pilgrim