Manava Dharmashastra 2.135

2.135

daśavarṣāṇi jātasya yasya sa bhavati 'daśavarṣaḥ' paricchedakaḥ kālaḥ, tasya paricchedyo brāhmaṇaḥ śrutaḥ | na ca tasyoccanīcatādi kārśyādi vā kālena parimātuṃ śakyam, kiṃ tarhi tadīyā kācitkriyā | sā ca janmanaḥ prabhṛti nityasamavāyinī prāṇadhāraṇalakṣaṇaiva |
evaṃ śatavarṣamiti |
pitāputrau tau draṣṭavyau |
tayoḥ sampradhāryamāṇayorbrāhmaṇaḥ pitā | ciravṛddhenāpi kṣatriyeṇa svalpavarṣo'pi brāhmaṇaḥ pratyutthāyābhivādyaśceti prakaraṇārthaḥ |

The Brāhmaṇa or ten years and the Kṣatriya or a hundred years should be known as father and son; and of the two this Brāhmaṇa is the father.—(135)

Medhātithi’s commentary (manubhāṣya):
One since whose birth ten years have elapsed is ‘of ten years.’ The time is the determining and the Brāhmaṇa the determined factor; but it is not possible to determine, by means of time, either his tallness or shortness or thinness, etc.; what can be determined by it is only a certain act of his; and this act can only be that of maintaining his life-breath, which subsists in him continuously since his birth.

The same explanation applies to the epithet ‘of a hundred years.’

They should be looked upon as ‘father and son.’

‘Of the two’—as compared with each other,—’ the Brāhmaṇa is the father.’

All that this means is that even when the Kṣatriya is very old and the Brāhmaṇa very young in years, the former should rise to meet and salute the latter.—(135)