Objection 1. It would seem that deficient age is not an impediment to marriage. For according to the laws children are under the care of a guardian until their twenty-fifth year. Therefore it would seem that before that age their reason is not sufficiently mature to give consent, and consequently that ought seemingly to be the age fixed for marrying. Yet marriage can be contracted before that age. Therefore lack of the appointed age is not an impediment to marriage.
Objection 2. Further, just as the tie of religion is perpetual so is the marriage tie. Now according to the new legislation (cap. Non Solum, De regular. et transeunt.) no one can be professed before the fourteenth year of age. Therefore neither could a person marry if defective age were an impediment.
Objection 3. Further, just as consent is necessary for marriage on the part of the man, so is it on the part of the woman. Now a woman can marry before the age of fourteen. Therefore a man can also.
Objection 4. Further, inability to copulate, unless it be perpetual and not known, is not an impediment to marriage. But lack of age is neither perpetual nor unknown. Therefore it is not an impediment to marriage.
Objection 5. Further, it is not included under any of the aforesaid impediments (50), and consequently would seem not to be an impediment to marriage.
On the contrary, A Decretal (cap. Quod Sedem, De frigid et malefic.) says that "a boy who is incapable of marriage intercourse is unfit to marry." But in the majority of cases he cannot pay the marriage debt before the age of fourteen (De Animal. vii). Therefore, etc.
Further, "There is a fixed limit of size and growth for all things in nature" according to the Philosopher (De Anima ii, 4): and consequently it would seem that, since marriage is natural, it must have a fixed age by defect of which it is impeded.
I answer that, Since marriage is effected by way of a contract, it comes under the ordinance of positive law like other contracts. Consequently according to law (cap. Tua, De sponsal. impub.) it is determined that marriage may not be contracted before the age of discretion when each party is capable of sufficient deliberation about marriage, and of mutual fulfilment of the marriage debt, and that marriages otherwise contracted are void. Now for the most part this age is the fourteenth year in males and the twelfth year in women: but since the ordinances of positive law are consequent upon what happens in the majority of cases, if anyone reach the required perfection before the aforesaid age, so that nature and reason are sufficiently developed to supply the lack of age, the marriage is not annulled. Wherefore if the parties who marry before the age of puberty have marital intercourse before the aforesaid age, their marriage is none the less perpetually indissoluble.
Reply to Objection 1. In matters to which nature inclines there is not required such a development of reason in order to deliberate, as in other matters: and therefore it is possible after deliberation to consent to marriage before one is able to manage one's own affairs in other matters without a guardian.
Reply to Objection 2. The same answer applies, since the religious vow is about matters outside the inclination of nature, and which offer greater difficulty than marriage.
Reply to Objection 3. It is said that woman comes to the age of puberty sooner than man does (De Animal. ix); hence there is no parallel between the two.
Reply to Objection 4. In this case there is an impediment not only as to inability to copulate, but also on account of the defect of the reason, which is not yet qualified to give rightly that consent which is to endure in perpetuity.
Reply to Objection 5. The impediment arising from defective age, like that which arises from madness, is reducible to the impediment of error; because a man has not yet the full use of his free-will.
Source: Summa Theologica (New Advent)