Article — Wouters, Manual of Moral Theology
Article I. Age
I. By natural law a determinate age is not required for the validity of marriage, but it suffices that someone has arrived at the use of reason, knows the nature of marriage sufficiently, and will have the power of coeundi in due time (St. Alphonsus VI).
II. By canon law, "a man before the completion of the sixteenth year of age, and a woman likewise before the completion of the fourteenth, cannot enter a valid marriage" (canon 1067, § 1). Years are to be taken as they are in the calendar; and since the terminus a quo is judged by law not to coincide with the beginning of the day, the first day is not to be counted, and the time is ended when the last day of the same number has been completed (canon 34, § 3).
But the impediment was established by the Church because she presumes that before the indicated years of age there is lacking at least the more mature discretion with which it is fitting to enter nuptials.
Corollary I. One who has attained the assigned age contracts validly, even if he or she does not yet have actual power for copula; by natural law the power which will be present in due time suffices. But the use of marriage is prohibited until each party has obtained actual power of coeundi, or at least there is hope of coeundi duly (St. Alphonsus, loc. cit.).
Corollary II. In doubt concerning age - which easily occurs in Mission regions - recourse should be had to the Ordinary, who can dispense in a doubt of fact. Sometimes the missionaries themselves receive the faculty of dispensing.
Corollary III. Although a marriage contracted after the aforesaid age is valid, nevertheless pastors of souls should take care to turn youths away from it before the age at which, according to the received customs of the region, marriage is usually entered (canon 1067, § 2). Therefore in colder regions it is rarely expedient that a young man contract nuptials before the twentieth year of age, or a girl before the eighteenth; indeed it is preferable to wait for the age of mature nubility, everywhere between the twentieth and twenty-fourth year, because otherwise spouses can harm themselves and their offspring. Once mature nubility has been obtained, one must of itself abstain from excessive delay of nuptials.