"Every one of the faithful has his own special guardian angel from baptism. (Sent. certa.)" (Ott, L. (1957). FCD. 120–121)
Furthermore, "According to the general teaching of the theologians, however, not only every baptised person, but every human being, including unbelievers, has his own special guardian angel from his birth. This view is biblically founded on the words of Our Lord. Mt. 18:10" (ibid)
"See that you despise not one of these little ones: for I say to you, that their angels in heaven always see the face of my Father who is in heaven." (Mt 18:10, Douay Rheims)
A look at the Early Churchʻs understanding of Mt 18:10 further reveals the Churchʻs teaching
“Therefore are they not to be despised for that they are so dear to God, that Angels are deputed to be their guardians” (St. Anselm)(Catena Aurea, Thomas Aquinas, S., & Newman, J. H. (1841).
“I esteem it, O my God, an inestimable benefit, that thou hast granted me an angel to guide me from the moment of my birth, to my death.” (St Augustine, De dilig. Deo. Medit. c. xii)( Haydock, G. L. (1859). Haydock's Catholic Bible Commentary)
Also the Angelic Doctor, St. Thomas Aquinas address the question of ʻwhether each man is guarded by an angel?ʻ, in his Summa Theologica
“Jerome says: Great is the dignity of souls, for each one to have an angel deputed to guard it from its birth.
I answer that, Each man has an angel guardian appointed to him. This rests upon the fact that the guardianship of angels belongs to the execution of Divine providence concerning men.” (STh., I q.113 a.2 s.c.–resp)
image from the 'Public domain' via wikimedia commons.The Guardian Angels. Pietro da Cortona