Complex sentence: It occurs only after receiving Baptism, and once the person has reached the age of reason (usually, around the second grade) first confession (the first sacrament of penance) must precede the first reception of the Eucharist.
Split into simpler sentences: It occurs only after receiving Baptism, and once the person has reached the age of reason (usually, around the second grade). First confession (the first sacrament of penance) must precede the reception of the Eucharist.