Psalm 92:14 describes the righteous bringing forth fruit in old age as a natural, expected thing: "they shall still bring forth fruit in old age; they shall be fat and flourishing." The Hebrew word for "fat" — dashen — means well-watered, rich, full of sap. It is the same word used to describe a well-fed, thriving plant. Old age, in this psalm, is not described as a diminishment of productive capacity but as a season of sustained flourishing.
Ecclesiastes 3:1 establishes a framework that holds the whole of life: "To every thing there is a season, and a time to every purpose under the heaven." Retirement is a season, not a conclusion. Every season in Ecclesiastes has a purpose — not just the seasons of building and planting but also the seasons of tearing down and mourning and rest. The question for retirement is not whether purpose is still available but what the purpose of this particular season is.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.