Adam Clarke on Proverbs 20:29

The glory of young men is their strength - Scarcely any young man affects to be wise, learned, etc.; but all delight to show their strength and to be reputed strong. Agility, one evidence of strength, their particularly affect; and hence their various trials of strength and fleetness in public exercises.

And the beauty of old men is the gray head - They no longer affect strength and agility, but they affect wisdom, experience, prudent counsels, etc., and are fond of being reputed wise, and of having respect paid to their understanding and experience.