The eagle lives in a place that cannot be easily reached by human beings, and the speaker is definitely aware of this. The poem imagines what the eagle's world is like, and by extension imagines a world without people. Still, the speaker can only describe the landscape using human or human-like attributes. Those darned "people" just keep popping up in the poem, like when you're looking at some cool geological formation and can't help thinking, "Hey, that looks just like my Aunt Gertrude!" "The Eagle" is a classic case of a nature poem that ends up being just as much about the person describing the scenery as anything else.
The poem compares the natural world to a person who experiences all stages of life at once: infancy, adulthood, and old age.
The speaker is incapable of imagining a non-human world. The poem's humanistic descriptions of nature are symptomatic of a society that views nature only as a means to achieve other things, not as an end in itself.