Today in India, the minimum age of marriage by law is at 18 years for women and at 21 for men. However, according to the official data, more than 12 millions [of which 84% are Hindus] are married below 10 years of age.

More than 125 million males were married before the legal age (42% of male population) and more than 102 million females (30% of female population) were married before the legal age in 2011. (source)

Historically, there was no minimum age for marriage in India throughout ages. Books considered sacred by Hindus point out that it was common for girls about 6 or 8 years old to be married.

The book Manu-smriti “Laws of Manu” that prescribes to Hindus their dharma (obligations) which is, according to Encyclopaedia Britannica, the most authoritative of the books of the Hindu code (Dharma-shastra) mentions in chapter IX (that concerned with issues related to marriages) a usual custom of marriage of girls 8 years old:
94. 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. (source)
The laws of Manu also don't forbid that the girl is married before she attains the age of puberty:
88. To a distinguished, handsome suitor (of) equal (caste) should (a father) give his daughter in accordance with the prescribed rule, though she have not attained (the proper age).
The Gautama's Dharmasūtra, which is believed to be the oldest of the four Hindu Dharmasastras, doesn't consider puberty a prerequisite for marriage:
21. A girl should be given in marriage before (she attains the age of) puberty.

22. He who neglects it, commits sin. (source)
Indian gods have many stories of early marriages; the Skanda Purana, the largest of the eighteen Mukyapurana [major puranas], tell us about the marriage of Rāma to a 6 years old girl:
8-9. The bow of Īśvara that was kept in the abode of Janaka, was broken. In his fifteenth year, O king, Rāma married the six-year old beautiful daughter of the king of Mithilā, Sītā who was not born of a womb. On getting Sītā, Rāghava became contented and happy. (source)
Another passage tells us about a 8 year old girl:
As time passed on she became a girl of eight years. The king recollected the words of the unembodied being and became worried. ‘To whom shall I give this daughter? Who will be the four-armed one?’......“Go ahead, O Kṛṣṇa, O mighty one. Let the jewel of a girl be seized quickly. I shall follow you closely behind causing much havoc unto all these demons.”.On getting the consent of Saṅkarṣaṇa, Keśava, the slayer of Keśin, seized the girl, immediately put her on the chariot and went off.(source)