Answer:
"A juxtaposition is when two or more things stand side by side, even though they usually are not associated with another."
Here is an example for an juxtaposition in your poem:
"THAT is no country for old men. The young / In one another's arms" (l.l.1)
The old men stands next to the young (-> old - young)
"Of what is past, or passing, or to come."
I think this might be an juxtaposition, too, but I am not sure. (the past next to the future)
I hope I could help you!