Answer: Then, and then only, have I thought how sweet / Old age might sink upon a windy youth.
In this poem, the author compares the arrival of the evening with the arrival of old age. She describes the way activity ceases when the light goes down, and this leads her to conclude that old age is similarly, the end of activity. The end of an active life, however, can arrive peacefully and sweetly to a "windy" (eventful) youth. This theme is reflected in the last option.