What is the reasoning behind delaying Confirmation until the age of reason - or for that matter delaying it for some until the age of eighteen?
it arose because historically in the western Church bishops reserved the right to confirm to themselves, and it as populations grew the bishop became more separated geographically from his flock, and could usually visit a given parish infrequently, and at that time would confirm all those who had been baptized. The whole history of conferring 1st communion is tied up with this, and the practice until about 100 years ago in most places was confirmation and 1st communion together at the age children normally ended their formal schooling, academic and religious, and went out to work, usually anywhere from about age 12 to 16 or 18. When the age of 1st communion was lowered to the age of reason, about 7, by Pope St. Pius X, confirmation was not formally addressed, so stayed at the older age in most places, kind of an orphan sacrament. Then the theology surrounding the preparation for the sacrament had to change so it focused on its role since the early Church of strengthening the Catholic for spiritual warfare, withstanding persecution, and using the gifts of the Holy Spirit to live the Christian life.
With the restoration of the RCIA and the original order of the sacraments, and the decree of the catechetical documents of the Church since V2 that sacramental initiation should follow the RCIA model, the entire issue of age for the sacraments, and hence preparation, is being re-examined. Since baptism at any age from infancy on up has been accepted throughout Church history, so should confirmation.
My personal opinion is it will take at least 100 years for the way the theology of the initiation sacraments is presented to change, and for the timing and preparation to catch up with the theology. Note, the theology won’t change, the emphasis will change and the “way we explain it to the parents.” The direction is definitely toward earlier initiation and restoration of the original order, so post-initiation catechesis will be restored to its intended mystagogical nature. This is in opposition to the current mentality, which sees CCD/PSR/RE as analagous to sacramental prep. For that to happen, all this generation of DREs has to die off first.