And millions in those solitudes, since first The flight of years began, have laid them down (55-56)
Here we get another big sweeping look back across the past. We’re not just talking about the last hundred years, or even a few millennia. The poem is rolling the clock all the way back to the beginning of time. It’s like we’re looking at an endless march of people lying down to die. Like so much of this poem, this image is kind of a funky mix. On the one hand, it’s sort of awe-inspiring to think of the millions and millions of people who went before us. On the other hand, it’s a real bummer – just a lot of death for ever and ever.
The youth in life’s green spring, and he who goes In the full strength of years, matron and maid, (68-69)
Now we zoom in a little. We’re not talking about stars and planets and the infinity of time and human history any more. Here we're talking about the stages of a human life. When you think about it, that’s just another way of measuring time. What the speaker is saying is that it doesn’t really matter that much whether you’re young or old, strong or frail. We’re all headed to the same place.