When the villain of the book, Randall Flagg, has captured a spy from the good guys in his home of Las Vegas, he tries to convince her that he isn't really going to attack the other settlement. With an amazing bit of logic he tells her that "If there is going to be a war, let our grandfathers fight it, or their grandfathers!"
I have been reading the section that you are talking about & Flagg was explaining to Dayna that if humanity goes on in 2190, then their
grandfathers to fight it out. That is, if they have a bone to pick
with each other. That means that if humanity survives by 2190 & they
(Flagg & the free zone) have a problem with each other, than they can
fight it out. Make sense?
As I recall Flagg had just killed the Judge, a very old man sent as a spy, much like Dana. He could have been commenting on the age of this spy since the whole reason for sending the old man had been because he was less suspicious. Flagg was probably making a joke about it.
"I have been reading the section that you are talking about & Flagg was explaining to Dayna that if humanity goes on in 2190, then their
grandfathers to fight it out."
Um. No. If humanity goes on until 2190, our grandCHILDREN would fight the war. It's a definite slip, not at all in the vein of Randall Flagg's jokes.