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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.