Commentaries:
There is a progression from sun to moon to stars in terms of the diminishing of light to a person observing from the earth. What does light represent in this context? Joy—the goodness of life. Solomon is asserting that, as a person ages, his capacity to enjoy the good things diminishes. He is underlining the inevitability of the problems of old age.
Even though Solomon directly says, "Remember now your Creator in the days of your youth," his meaning is more emphatic: "Get started now! Do not wait! Do not delay! Live life the right way when you are young because, as you get older, your capacity to enjoy diminishes."
John W. Ritenbaugh
Ecclesiastes and the Feast of Tabernacles (Part 2)