Manava Dharmashastra 2.156

2.156

na tena vṛddha ucyate yenāsya palitaṃ dhavalaṃ śiraḥ śiraḥsthāḥ keśāḥ | kathaṃ tarhi yo vai yuvā'pi taruṇo'pi atha cādhīte taṃ devāḥ sthaviraṃ viduḥ bruvate | devāḥ kila sarvasya veditāra iti praśaṃsā |

One does not become venerable by the fact that his hair has turned grey; the gods know him to be venerable who, though young, continues to study.—(156)

Medhātithi’s commentary (manubhāṣya):
One is not called ‘venerable by the fact that his head has turned grey’;—i.e., the hairs of his head have become white. How then?

He who, ‘though young’—is of young age—and yet carries on his study,—him ‘the gods know’—declare—‘to be venerable.’ The gods know all things, hence this is a praise (of the learned man).—(156)

Comparative notes by various authors
Gautama (see under 154).