What does the speaker in On My First Son compare years of his short life to?

In the poem "On My First Son," the speaker compares the years of his son's short life to "dark ages." He says that the son's life was like "A thousand dark ages, where they lie / Steeped in continual darkness." This comparison emphasizes the brevity of the son's life and the speaker's sense of loss and grief. The speaker also uses the metaphor of "dark ages" to suggest that his son's life was a time of suffering and hardship.