Manava Dharmashastra 2.212

2.212

pūrṇaviṃśativarṣeṇa taruṇenetyarthaḥ | bālasya ā ṣoḍaśādvarṣādadoṣaḥ | pūrṇāni viṃśati varṣāṇi yasya sa evamucyate | ayaṃ kālo yauvanodbhedopalakṣaṇārthaḥ | ata evāha guṇadoṣau vijānatā | kāmaje sukhaduḥkhe guṇadoṣāvabhipretau strīgatau ca svākṛtidurākṛtilakṣaṇau dhairyacāpale vā | sarvathā'tantrā viṃśatisaṃkhyā |

Medhātithi’s commentary (manubhāṣya):
‘One who is full twenty years old’;—i.e., fully grown up. There is no harm in the case of the pupil who is still a ‘child,’ not having passed his sixteenth year. What is meant is one who has completed his twenty years. To the same effect we have the next qualification—‘who is conscious of what is good and what is bad.’ The ‘good’ and ‘bad’ meant here are the pleasures and pains arising from sexual love, also the beauty and ugliness of women, as also their fidelity and infidelity.

In any case stress is not meant to be laid upon the number ‘twenty.’—(212)