I remember reading that girls should be married within the age of 12. The intention of the question is not to incite a debate on the ideal age for marriage in today's world, but to know what our scriptures recommend.
Here is what this chapter of the Manu Smriti recommends:
- A man, aged thirty years, shall marry a maiden of twelve who pleases him, or a man of twenty-four a girl eight years of age; if (the performance of) his duties would (otherwise) be impeded, (he must marry) sooner.
Bhishma says much the same thing in this chapter of the Anushasana Parva of the Mahabharata:
A person of thirty years of age should wed a girl of ten years of age called a Nagnika. Or, a person of one and twenty years of age should wed a girl of seven years of age.
Note that these recommendations are more binding for the lower bound rather than the upper bound. Here is what the Mahabharata translator says in a footnote:
Nagnika is said to be one who wears a single piece of cloth. A girl in whom the signs of puberty have not appeared does not require more than a single piece of cloth to cover her. The mention of Nagnika, the commentator thinks, is due to an interdiction about wedding a girl of even ten years in whom signs of puberty have appeared.
From kamakoti.org - Kanchi Periyavar - "Pradanam prak rtuh" - before puberty, which is normally around age 12 for girls.
There is also a prayaschittam for performing after puberty. You can ask your brihaspati/purohit.
The aim is to ensure that the foundation of marriage is based on innocent attraction and attachment before kama/carnal desire enters mind. In olden days, the concept of 'child marriage' did not mean 'child physical relations' which is what most people who deride it today think of.
The bride would only be sent to groom's house after puberty. Source - personal family ancestors.
I don't have any opinion for this Qn. Just to be clear that, "marriage" doesn't necessarily mean "sex" with a girl. It's more of a mental bonding, where girl starts seeing her husband as her protector and master.
Presenting a reference from Mahabharata AnushAsana parva:
A person of thirty years of age should wed a girl of ten years of age called a Nagnika.1 Or, a person of one and twenty years of age should wed a girl of seven years of age. That girl who has no brother nor father should not be wed, O chief of Bharata's race, for she may be intended as Putrika of her sire.2 After the appearance of puberty, the girl (if not married) should wait for three years. On the fourth year, she should look for a husband herself (without waiting any longer for her kinsmen to select one for her). The offspring of such a girl do not lose their respectability, nor does union with such a girl become disgraceful. If, instead of selecting a husband for herself, she acts otherwise, she incurs the reproach of Prajapati herself. One should wed that girl who is not a Sapinda of one's mother or of the same Gotra with one's father. Even this is the usage (consistent with the sacred law) which Manu has declared.'3
1: Nagnika is said to be one who wears a single piece of cloth. A girl in whom the signs of puberty have not appeared does not require more than a single piece of cloth to cover her. The mention of Nagnika, the commentator thinks, is due to an interdiction about wedding a girl of even ten years in whom signs of puberty have appeared.
2 When a father happens to have an only daughter, he frequently bestows her in marriage upon some eligible youth on the understanding that the son born of her shall be the son, for purposes of both Sraddha rites and inheritance, not of the husband begetting him but of the girl's father. Such a contract would be valid whether expressed or not at the time of marriage. The mere wish of the girl's father, unexpressed at the time of marriage, would convert the son into a son not of the father who begets him but of the father of the girl herself. A daughter reserved for such a purpose is said to be a putrikadharmini or 'invested with the character of a son.' To wed such a girl was not honourable. It was in effect an abandonment of the fruits of marriage. Even if dead at the time of marriage, still if the father had, while living, cherished such a wish, that would convert the girl into a putrikadharmini. The repugnance to wedding girls without father and brothers exists to this day.
3 For understanding the meanings of Sapinda and Sagotra see any work on Hindu law civil or canonical.
According to the Vishnu Purana, it is recommended for grooms to choose brides who are a third of their age:
If he must marry, he must select a maiden who is third of his age, one who has not too much hair but is not without any, one who is not very black nor yellow complexioned and who is not from birth a cripple or deformed. (Vishnu Purana, Book III, Chapter X)