"No Country for Old Men", why is it called so?

I know this has been answered and accepted, but I'm not sure this can be answered completely without going to the poem from which the book gets its title, Sailing to Byzantium by Wm. Butler Yeats. The first line is

That is no country for old men.

"That" is anywhere in the world - any country, full of youth and things that live and die without a thought about the meaning of life, where modernism (a post-WWI movement; is there a meaning to life?) has taken hold, a place without respect for tradition and timeless things - That is no country for old men.

And therefore I have sailed the seas and come
To the holy city of Byzantium.

Byzantium is an ageless and golden city, of art and sages and "God's holy fire". The poet (in his sixties when he wrote this) is tired of being old and of holding values different from those in his surroundings. He's ready to pass beyond life into something eternally valuable (in this case, art).

Consume my heart away; sick with desire
And fastened to a dying animal
It knows not what it is; and gather me
Into the artifice of eternity.

So, too, the musings of the protagonist (Sheriff Ed Tom Bell) throughout the story show that he is living in and confronted by a "modern" world which holds no respect for traditional values - reason, respect for life, even common decency - a basis for law, which was present in the old days, why lawmen didn't need to carry guns 'back then'.

Modernism was a reaction to the senseless slaughter of millions in WWI through modern warfare, which changed the world, and not for the better. This may be why Bell speaks so often of the old timers not using guns. All the loss of 'back then' is exactly what Yeats is mourning. Is the senseless and brutal drug war in the story the counterpart here to WWI?

This modern world is no country for old men, and the poem is again alluded to when in the last monologue, Sheriff Bell describes two dreams, both with his father in them. In the first he casually mentions losing money (rejecting today's materialism? I mean, the whole movie is about keeping and chasing money.) In the second, he's leaving the modern times behind; he's taking a trip on horseback in the night, going through a mountain pass (the mountains mentioned twice, a metaphor for the barrier between life and death? Is he sailing to Byzantium?) His father is going ahead of him - his father preceded him in death - carrying fire, again mentioned twice, and described as "the color of the moon". God's holy fire?

To me, the whole story is a reflection of the very poem the title comes from.

_{Oops. In looking for support for my theory, I just found this: Yeats' "Sailing to Byzantium" and McCarthy's "No Country for Old Men": Art and Artifice in the New Novel. I haven't read it yet, but it should be interesting.}