Temperature is a powerful metaphor in this poem for youth, health, vigor, and the like.
You know how when you first take something out of the microwave it's really hot? Now imagine human life as that microwave burrito. When you're young – i.e., just out of the microwave – you're still hot, but as you get older you get colder (hey that rhymes!). Who likes a cold burrito anyway?
- Line 5: The speaker calls the sun a "glorious lamp." "Lamp" is a metaphor for the sun, which lights up the sky just like a lamp. Both "lamp" and "sun" suggest warmth.
- Line 8: When the sun sets, the temperature drops. "Setting" is here a metaphor for what appears to happen at the end of the day. Also, "setting" is a human activity, and the sun isn't human; this is called personification.
- Lines 9-10: The speaker calls youth the best "age." People aren't literally "warmer" when they're younger, so "warmer" is here a metaphor for health, vigor, and other things we associate with youth.