Bridges on Proverbs 20:29
Charles Bridges on Proverbs 20:29

29.  The glory of young men is their strength: and the beauty of old men is the gray head.
Every stage of life has its peculiar honor and privilege. ‘Youth is the glory of nature, and strength is the glory of youth. Old age is the majestic beauty of nature, and the grey head is the majestic beauty, which nature hath given to old age.’† Yet these pictures describe the use, not the abuse. It is their youth usefully exercised, especially consecrated to God, and employed for his glory. Otherwise, as an occasion of wantonness,† or vain-glorious boast,† its strength is its shame, and will end in vanity.† The silver crown brings honour, and reverence, and authority — only “in the way of righteousness.”†
Yet the beauty of the grey head is most likely to be found, where the strength and glory of youth have been dedicated to God. The young plant, stunted, and deformed in its youth, will generally carry its crookedness into advancing growth. But who can calculate upon the extent of fruitfulness, where “the beginning of our strength” — “the dew of our youth” — has been given to the LORD? (Psalm 92:13-15.) Let youth and age however each beware of defacing their glory. Each takes the precedence in some things, and gives place in others. Let them not therefore envy or despise each other’s prerogatives. The world — the state — the church needs them both — the strength of youth for energy, and the maturity of age for wisdom.