There exists a generally accepted doctrine which says that any period of the unmarried daughter without intercourse is a crime, a killing of the potential embryo (THIEME 444, SYED To 118). The term for this 'crime' is bhrunahatya, killing of the embryo: bhruna = embryo, hatya = killing (WEZLER Bh 627-628). According to tradition, the criminal character of the deed is apparent from the blood of the menstruation. In the case of omitted intercourse the family's pitris drink the menstrual blood of the girl. This is, however, not clearly communis opinio. The beginning of the macabre motif cannot be dated (ABEGG Pretakalpa 247; KANE 444; SYED To 118). The Pretakalpa, which contains an early reference to this strange belief, must be comparatively late since it already mentions the suttee (ABEGG 18-19). There is earlier evidence of the hungry pitris, but even then the motif cannot be very old. The rule of early marriage of girls is certainly earlier than the motif of the blood drinking pitris and perhaps even earlier than the (more general) bhrunahatya doctrine. It started apparently after Manu and was in the first stage (when?) restricted to Brahman girls. Still later (when?) non-Brahmans were also included. KANE 442-452.
Whatever the belief in the idiosyncrasies of mythical ancestors, it is a matter of prestige, reasons not absolutely clear, to arrange in time a marriage for one's daughter. To have pubescent but still unmarried daughters in the house is disgraceful for the family. -- The future husband has to cohabit with his wife every month. See SYED To 118-119.
We add a few mutually divergent opinions. ALTEKAR observes "that marriage in the Vedic age took place when the parties were fully grown up." (51) JOLLY says about the following (later) time as far as reflected in the law-books: "The rule that the marriage must precede the puberty and that the bride must be nagnika [naked] is common to all smritis" (56). A different matter is the reinterpretation of nagnika by THIEME. According to him, nagnika means "not yet but soon in the marriageable age." (443). This would imply a marriage shortly before the menarche (not earlier). Nagnika may also have the literal meaning 'naked'. At least in one case very early marriage (true nakedness) is prescribed, the term nagnika being clearly implied (THIEME 442). The early Dharma Shastras (the later Dharma Shastras anyway) thus suggest an early age (early or very early, THIEME 441) which is not in keeping with the view of the historians who ascribe the lowering of the age to a later period. ALTEKAR describes the gradual reduction of the marriage age of girls and writes "From about 200 A.D., pre-puberty marriages became the order of the day." (56) On the other hand "Sanskrit poets and dramatists [200-800 A.D.?] always depict that the heroines in their works are grown-up at the time of marriage..." (KANE 446). The same applies to the epics. There is thus an unsolved conflict between Dharma Shastra and literature (poetry and epics).
In the Manu Smriti and elsewhere in the Smriti Literature the age of marriage was expressed in terms of years, but not in unequivocal form. There is a directive that a bride should be much younger than the bridegroom (Manu 9.94 has 30:12 and 24:8); THIEME 436-437. There is another rule that in case of problems a girl may be married three years after the begin of puberty, but there is the corresponding rule that a girl should be married three months after the begin of puberty (ALTEKAR 53-54; THIEME 437-438). A radical formula prescribes for the girl 4-6 as the minimum and 8 as the maximum of years (JOLLY 56). Post-Christian medical texts recommend the quasi-realistic ratios of 21:12 and 25:16 (WINTERNITZ 33). The Brahma Purana suggests a marriage-age between 4 and 10 (THIEME 441-442). Refer for the lack of uniformity in dharmic prescriptions to the Glossary (dharma). -- JOLLY 56-57, WINTERNITZ 32-35, ALTEKAR 49-65, SCHMIDT 78-80.
We add a verse by the poet Bana (A.D.606-647): "At the time when her breasts become visible, a daughter, growing from year to year [and still living with her parents], hurls her father into the whirlpool of despair, just as a river destroys its bank in the whirlpool of its current" (SYED To 117, § 8.8). Here the marriageable girl is no longer a true nagnika and not yet an adult female (as in the epic). ALTEKAR demonstrates in one case the social difference between pre-puberty and post-puberty marriages. He says: "In the Deccan also during the Vijayanagar rule [1336-1648] while pre-puberty marriages were common among the Brahmanas, post-puberty ones were frequent among the non-Brahmanas [Marathas?]." (ALTEKAR 58)
Refer for the entire complex of early marriage to THIEME 435-445. -- The marriage age of a son is not subject to precise rules. -- The problem of unmarried mothers (Europe!) obviously did not exist.
The old marriage ceremonies (existing from Vedic days to the present) suit the requirements of more or less mature couples. When 'child marriages' had come to stay, a second marriage was introduced, so that the (second) marriage took place when the girl (girl and boy) had reached the necessary age. Before that event, and after the first marriage, the future bride stayed in the house of her parents. JOLLY 56: "fresh ceremonies at the beginning of married life" (the fresh ceremonies and the early ceremonies are not described separately, however).
In the Manu Smriti and perhaps in all law-books, the eldest son is the only son that matters (MALAMOUD De 52, fils ainé). This rule makes a consideration of the other possible cases (eschatological role of a second son, of an adopted son) less relevant. There nevertheless existed the obvious wish to have more than one son (SUTHERLAND 88).
The negative medical consequences of the early marriage of women have been studied in modern times (WINTERNITZ 27-36; MAYO Ch.3-5). See also SYED To 111-116.
§§ 4-12 must be taken for what they are worth -- strictly speaking we do not know 'what actually happened' in ancient India (§ 5 supra). On the one hand we do not know how far Manu (or any Dharma Shastra) was taken into consideration in real life, on the other hand it is not clear which parts of India were in the course of history under the direct control of Hindu kings and which were not. Were parts of India 'non-Hindu' or 'semi-Hinduized'? The subject of ignorance of the past recurs in various contexts. There is the old saying that we do not know what in the past every fish-wife knew.