Greetings!
I’m not sure in particular about the EF, but Confirmation had been practiced in one liturgical setting in the west with Baptism until the 11th century or so. In the west, the bishop was the ordinary minister of the Sacrament of Confirmation. Because people were spread out it took time for the bishop to Baptize and Confirm infants in his “geographical” area. This caused a delay in the Confirmation of infants so they would end up being Confirmed a little while after their Baptism, whenever the bishop had the opportunity to be there. The eastern Churches, both Catholic and Orthodox, continue this tradition of Baptism and Confirmation together in one liturgical setting.
It wasn’t until around the turn of the century that children were Confirmed at a much later age in the west.
ZP