C. F. Keil and Franz Delitzsch on Proverbs 20:29

Pro 20:29 29 The ornament of young men is their strength;      And the honour of the old is grey hairs.

Youth has the name בּחוּר (different from בּחוּר, chosen), of the maturity (R. בחר, cogn. בכר, בגר, whence Mishn. בּגרוּת, manhood, in contradistinction to נערוּת) into which he enters from the bloom of boyhood; and the old man is called זקן (Arab. dhikn, as Schultens says, a mento pendulo, from the hanging chin זקן, (Arab.) dhakan, chin, beard on the chin). To stand in the fulness of fresh unwasted strength is to youth, as such, an ornament (תּפארת, cf. פּארוּר, blooming colour of the countenance); on the contrary, to the old man who has spent his strength in the duties of his office, or as it is said at Pro 16:31, “in the way of righteousness,” grey hairs (שׂיבה, from שׂב, Arab. shâb, canescere) give an honourable appearance (הדר, from הדר, turgidum, amplum esse, vid., at Isa 63:1).