Below are some additional indications that could be followed up in the course of your research.
Kind regards,
WS
1) See Richard Schmidt (Beiträge zur indischen Erotik. 3. Aufl. Berlin 1922: 645–649) with source quotes on marriage age and also marks of pubescence).
2) See moreover Ram Gopal, India of Vedic Kalpasūtras. Delhi 21983: 212 with relevant quotes (p. 220, n. 59) also on the important term nagnikā („naked“) in the context of the ideal marriage age:
nagnikām [=] aprāptastrībhāvām ayauvanarasām upayaccheta (“let him approach a nagnikā girl for intercourse in whom the sexual characteristics of a woman are not yet developed and in whom the menstrual fluid (yauvanarasa) has not yet emerged.”).
nagnikā, defined as the “best” (śreṣṭhā) in the above passage of Mānavagṛhyasūtra as cited by Gopal, seems actually to refer to the absence of pubic hair (ajātalomnī) as also discussed, e.g., by Bhaṭṭanārāyaṇa on Gobhilagṛhyasūtra 2.5.7. According to the latter’s testimony there were Ācāryas who stipulated intercourse with prepubescent married girls lacking pubic hair, if these girls themselves desired so:
yady ajātalomny evātīva puruṣābhogārthinī syāt, tathā sati, […] maithunaṃ kartavyam ity eke ācāryā manyante.
3) The Kashmirian Kāṭhakagṛhyasūtra determines the age of marriage of girls at 10, at the very latest at 12 years (daśavārṣikaṃ brahmacaryaṃ kumārīṇāṃ dvādaśavārṣikaṃ vā KGS 19.2), on which Devapāla comments: varṣadaśakād ūrdhvaṃ brahmacarye kumārī na sthāpayitavyā pitrā । agatyāvā dvādaśa varṣāṇi nātikramaṇīyāni ॥ (Devapālabhāṣya ad 19.2. ||
4) Manusmṛti
triṃśadvarṣo vahet kanyāṃ hṛdyāṃ dvādaśavārṣikīm |
tryaṣṭavarṣo 'ṣṭavarṣāṃ vā dharme sīdati satvaraḥ || MDhŚ 9.94 ||
“A 30-year-old man should marry a charming girl of 12 years, or an 18-year-old, a girl of 8 years - sooner, if his fulfilling the Law would suffer.” (Olivelle 2005, p. 194).
There is a wider range of evidence for an ideal marriage age for girls aged 8 (aṣṭavarṣā): To start with Pārvatī, Śiva’s wife, it is said that she was married at the age of eight (8), i.e. before puberty, the technical term for which is gaurī (significantly also used as an epithet for her):
5) Jayadratha’s Haracaritacintāmaṇi
devī himavataḥ putrī kālī nīlotpalacchaviḥ |
aṣṭavarṣā tapoyuktā bhartāraṃ prāpa dhūrjaṭim || Hc 22.3 ||
sā krīḍantī pitṛgehe śambhunā saha pārvatī |
dṛṣṭvā dṛṣṭvā vapuḥ śyāmaṃ nāhaṃ gaurīty alajjata || Hc 22.4 ||
6) gaurī = aṣṭavarṣā = prepubescent:
Brhadyamasmrti (= Parāśarasmṛti 7.4):
aṣṭavarṣā bhaved gaurī navavarṣā ca rohiṇī |
daśavarṣā bhavet kanyā ata ūrdhvaṃ rajasvalā || YS 182v 3.21 ||
7) Aṣṭavarṣā marriage in the Revākhaṇḍa of the Vāyupurāṇa:
puṇyāham adya saṃjātam ahaṃ tvaddarśanotsukaḥ |
kanyā madīyā rājendra hy aṣṭavarṣā vyajāyata || RKV 142.18 ||
[…]
caturbhujo mama sutas triṣu lokeṣu viśrutaḥ |
tasyeyaṃ dīyatāṃ kanyā śiśupālasya bhīṣmaka || RKV 142.20 ||
8) Rāmāyaṇa
Sītā, too, was married before the age of puberty as a “kaumārī”:
svayaṃ tu bhāryāṃ kaumārīṃ ciram adhyuṣitāṃ satīm |
śailūṣa iva māṃ rāma parebhyo dātum icchasi || Rām 2.27.8 (CE)||
The Gītā Press translates from an emic insight point of view: „who was married to you before puberty”.
The commentaries Rāmāyaṇaśiromaṇi und Bhūṣaṇa on this passage (Rām 2.30,8) confirm kaumārī as “kumārāvasthāyāṃ eva vivāhitā” (“married already in the period of life of a ten to twelve years old maiden”).
kumārī = 1. “A young girl, one from 10 to 12 years old“ (Apte)
9) A significant term for a a sexually mature, fully developed girl is prauḍhā (cp., eg., Bhāgavatapurāṇa 4.25.21 (a-prauḍhā – “not yet fully developed”), orūḍhā (cp. nava-ūḍhā – “having just attained puberty”, as in Brahmavaivartap., ch. 112).