At what age should Christians be confirmed according to St. Thomas Aquinas? I've read this in Saint Thomas Aquinas' sermons on the Apostles' Creed (emphasis mine):


  “Confirmation.”—The second Sacrament is Confirmation. Just as they who
  are physically born need certain powers to act, so those who are
  reborn spiritually must have the strength of the Holy Spirit which is
  imparted to them in this Sacrament. In order that they might become
  strong, the Apostles received the Holy Spirit after the Ascension of
  Christ: “Stay you in the city till you be endowed with power from on
  high” [Lk 24:49]. This power is given in the Sacrament of
  Confirmation. They, therefore, who have the care of children should be
  very careful to see that they be confirmed, because great grace is
  conferred in Confirmation. He who is confirmed will, when he dies,
  enjoy greater glory than one not confirmed, because greater grace will
  be his.


Thomas speaks about "children" to be confirmed. The common practice today is to confirm young adults, to mark their maturity and mature acceptance of their faith. Thomas seems to advocate the other concept of the sacrament of confirmation: this sacrament should open the young boy or girl to the help of Holy Spirit to become a true adult Christian.

When should young Christian be confirmed according to st. Thomas? Or in other words: what were the common ages when people were confirmed in 13th century? Of these, it's easy to pick those that can be seen as "children".
By St Thomas's time, infant baptism would have been the norm for many centuries. However, confirmation has always had to fight for survival, in that it is not necessary for salvation and therefore once it was separated from baptism it was hard to get people to bring their children of any age to receive it on the rare occasions when the bishop happened to be available. (The word children can refer to relationship without restriction to age.) There is no set age when a person receives confirmation, either in St Thomas's day or our own. It depends not only on the readiness of the child/adult but also the availability of the bishop, who is normally the one who administers the sacrament since at least the second council of Lyons in 1274. Often a bishop has historically not been able to visit every parish in his see in a year, or even several years, especially in the days before modern roads. In St. Thomas's time most parishes probably didn't give much, if any, catechesis to prepare people for confirmation either, so it didn't matter so much what age they were when they received it. Education of any kind was a rarity.

In the Latin rite today, after infant baptism, childhood first communion (and first confession) precedes confirmation, usually by several years. No episcopal presence is needed for these earlier initiation rites. Simplified catechesis takes place before first communion, and more complete catechesis precedes confirmation. Of course, for adults in RCIA, baptism, confirmation and communion can all take place in a single ceremony at the Easter vigil, and is preceded by several months of adult catechesis.

A good reference for the history of the sacraments is "Doors to the Sacred" by Joseph Martos.