Roman Catechism 22.10

Chapter 22 — Part II: The Sacraments — Matrimony

Section 10. Next follow the words between qualified persons; for persons excluded by law cannot contract marriage, and if they do their marriage is invalid. Persons, for instance, within the fourth degree of kindred, a boy before his fourteenth year, and a female before her twelfth, the ages established by law, cannot contract marriage.