The Catechism of the Catholic Church tells us “Angels are spiritual creatures who glorify God without ceasing” (no.350). Although the Church does not have a formal teaching on guardian angels, it is a traditional belief that every person has their own particular angel. The Church has always held the belief, even designating October 2 as the Feast of the Guardian Angels. The Catechism of the Catholic Church, no. 336, states:
From its beginning until death human life is surrounded by their watchful care and intercession. “Beside each believer stands an angel as protector and shepherd leading him to life” [quoting St. Basil in Adv. Eunomium III, I]. Already here on earth the Christian life shares by faith in the blessed company of angels and men united in God.
God does not create a new angel for each new baby. Angels were created sometime before the creation of the world; they “have been present since creation and throughout the history of salvation” (Catechism no. 332). They are created immortal beings, meaning that they are created (not eternal, like God) and they cannot die. Angels are not separated from us after death, but remain with us in heaven, not to help us attain salvation, but ad aliquam illustrationem (to enlighten us with their angelic ministry) (St. Thomas Aquinas, Summa Theologica).