Final answer:
The phrase "when dawns were young" in line 5 of the poem "The Negro Speaks of Rivers" indicates that the time period being discussed is when the earth was still new.
Step-by-step explanation:
The phrase "when dawns were young" in line 5 of the poem "The Negro Speaks of Rivers" indicates that the time period being discussed is when the earth was still new. This phrase suggests that the speaker is referring to a very ancient period in history, before significant human presence. It helps convey the idea of timeless existence and the enduring nature of the rivers that the speaker mentions throughout the poem.