When little lights in little ports come out,
Quivering down through water with the stars,
And all the fishing fleet of slender spars
Range at their moorings, veer with tide about;

When race of wind is stilled and sails are furled,
And underneath our single riding-light
The curve of black-ribbed deck gleams palely white,
And slumbrous waters pool a slumbrous world;

Then, and then only, have I thought how sweet
Old age might sink upon a windy youth,
Quiet beneath the riding-light of truth,
Weathered through storms, and gracious in retreat.

How do the lines in bold develop the theme of the poem?