The affirmation of belief is not a part of the Sacrament of Confirmation by its nature (otherwise the Confirmation of infants in the majority of Catholic Churches would be invalid

). By delaying the Sacrament as is common practice, we make it necessary that the Confirmandi are able to give informed consent.
It is a historical anomaly that we allow non-Confirmed to approach the altar for Communion, this has only been allowed in the West, as an exception starting in the 1930’s. Not until later in the century was it normal to have Confirmation follow Communion (and even then 4th grade seemed to develop as the custom through the 1960’s). Only since 1983 and the new code of Canon Law has the creeping age of Confirmation reached the absurd point where people wonder if 14 or even 18 is too young. The creeping age distorted the understanding of the Sacrament into that of a “Catholic Bar/Bat Mitzvahs”. The Sacraments are not rights of passage, and this notion leads to other problems (ie people “graduating” from Church).
The graces of Confirmation are to aid us in perseverance in the Truth and against sin. At the latest these should be received when the capacity for mortal sin exists, that is the age of reason. As of that point, baptized Catholics are bound to the life of the Church, again why deny them the graces given through the Sacraments to fulfill their obligations?