Isaiah 46:4 is God's most direct promise about old age in Scripture: "even to your old age I am he; and even to hoar hairs will I carry you: I have made, and I will bear; even I will carry, and will deliver you." The hoar hairs — grey or white hair — are explicitly named as the condition under which God carries. The verb nasa means to bear a load, to carry a burden. God is not describing a distant watch over aging people; he is describing himself taking on their weight as their own capacity decreases.
Psalm 92:14 says the righteous "shall still bring forth fruit in old age." The image is of a tree still producing — not a retired tree that has given what it will give. The flourishing is described as a condition of the aged righteous, not merely a possibility for the exceptional ones.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.