Objection 1. It would seem that seven years is not fittingly assigned as the age for betrothal. For a contract that can be formed by others does not require discretion in those whom it concerns. Now a betrothal can be arranged by the parents without the knowledge of either of the persons betrothed. Therefore a betrothal can be arranged before the age of seven years as well as after.
Objection 2. Further, just as some use of reason is necessary for the contract of betrothal, so is there for the consent to mortal sin. Now, as Gregory says (Dial. iv), a boy of five years of age was carried off by the devil on account of the sin of blasphemy. Therefore a betrothal can take place before the age of seven years.
Objection 3. Further, a betrothal is directed to marriage. But for marriage the same age is not assigned to boy and girl.
Objection 4. Further, one can become betrothed as soon as future marriage can be agreeable to one. Now signs of this agreeableness are often apparent in boys before the age of seven. Therefore they can become betrothed before that age.
Objection 5. Further, if persons become betrothed before they are seven years old, and subsequently after the age of seven and before the age of maturity renew their promise in words expressive of the present, they are reckoned to be betrothed. Now this is not by virtue of the second contract, since they intend to contract not betrothal but marriage. Therefore it is by the virtue of the first; and thus espousals can be contracted before the age of seven.
Objection 6. Further, when a thing is done by many persons in common, if one fails he is supplied by another, as in the case of those who row a boat. Now the contract of betrothal is an action common to the contracting parties. Therefore if one be of mature age, he can contract a betrothal with a girl who is not seven years old, since the lack of age in one is more than counterbalanced in the other.
Objection 7. Further, those who at about the age of puberty, but before it, enter into the marriage contract by words expressive of the present are reputed to be married. Therefore in like manner if they contract marriage by words expressive of the future, before yet close on the age of puberty, they are to be reputed as betrothed.
I answer that, The age of seven years is fixed reasonably enough by law for the contracting of betrothals, for since a betrothal is a promise of the future, as already stated (1), it follows that they are within the competency of those who can make a promise in some way, and this is only for those who can have some foresight of the future, and this requires the use of reason, of which three degrees are to be observed, according to the Philosopher (Ethic. i, 4). The first is when a person neither understands by himself nor is able to learn from another; the second stage is when a man can learn from another but is incapable by himself of consideration and understanding; the third degree is when a man is both able to learn from another and to consider by himself. And since reason develops in man by little and little, in proportion as the movement and fluctuation of the humors is calmed, man reaches the first stage of reason before his seventh year; and consequently during that period he is unfit for any contract, and therefore for betrothal. But he begins to reach the second stage at the end of his first seven years, wherefore children at that age are sent to school. But man begins to reach the third stage at the end of his second seven years, as regards things concerning his person, when his natural reason develops; but as regards things outside his person, at the end of his third seven years. Hence before his first seven years a man is not fit to make any contract, but at the end of that period he begins to be fit to make certain promises for the future, especially about those things to which natural reason inclines us more, though he is not fit to bind himself by a perpetual obligation, because as yet he has not a firm will. Hence at that age betrothals can be contracted. But at the end of the second seven years he can already bind himself in matters concerning his person, either to religion or to wedlock. And after the third seven years he can bind himself in other matters also; and according to the laws he is given the power of disposing of his property after his twenty-second year.
Reply to Objection 1. If the parties are betrothed by another person before they reach the age of puberty, either of them or both can demur; wherefore in that case the betrothal does not take effect, so that neither does any affinity result therefrom. Hence a betrothal made between certain persons by some other takes effect, in so far as those between whom the betrothal is arranged do not demur when they reach the proper age, whence they are understood to consent to what others have done.
Reply to Objection 2. Some say that the boy of whom Gregory tells this story was not lost, and that he did not sin mortally; and that this vision was for the purpose of making the father sorrowful, for he had sinned in the boy through failing to correct him. But this is contrary to the express intention of Gregory, who says (Dial. iv) that "the boy's father having neglected the soul of his little son, fostered no little sinner for the flames of hell." Consequently it must be said that for a mortal sin it is sufficient to give consent to something present, whereas in a betrothal the consent is to something future; and greater discretion of reason is required for looking to the future than for consenting to one present act. Wherefore a man can sin mortally before he can bind himself to a future obligation.
Reply to Objection 3. Regarding the age for the marriage contract a disposition is required not only on the part of the use of reason, but also on the part of the body, in that it is necessary to be of an age adapted to procreation. And since a girl becomes apt for the act of procreation in her twelfth year, and a boy at the end of his second seven years, as the Philosopher says (De Hist. Anim. vii), whereas the age is the same in both for attaining the use of reason which is the sole condition for betrothal, hence it is that the one age is assigned for both as regards betrothal, but not as regards marriage.
Reply to Objection 4. This agreeableness in regard to boys under the age of seven does not result from the perfect use of reason, since they are not as yet possessed of complete self-control; it results rather from the movement of nature than from any process of reason. Consequently, this agreeableness does not suffice for contracting a betrothal.
Reply to Objection 5. In this case, although the second contract does not amount to marriage, nevertheless the parties show that they ratify their former promise; wherefore the first contract is confirmed by the second.
Reply to Objection 6. Those who row a boat act by way of one cause, and consequently what is lacking in one can be supplied by another. But those who make a contract of betrothal act as distinct persons, since a betrothal can only be between two parties; wherefore it is necessary for each to be qualified to contract, and thus the defect of one is an obstacle to their betrothal, nor can it be supplied by the other.
Reply to Objection 7. It is true that in the matter of betrothal if the contracting parties are close upon the age of seven, the contract of betrothal is valid, since, according to the Philosopher (Phys. ii, 56), "when little is lacking it seems as though nothing were lacking." Some fix the margin at six months. but it is better to determine it according to the condition of the contracting parties, since the use of reason comes sooner to some than to others.
Source: Summa Theologica (New Advent)