Wasn't it said somewhere that the point of divergence occured when JFK wasn't assassinated?
Maybe I'm just imagining that though.
Would be the first I ever heard of that.
We were not really given any info on when it really stopped.
We do know it was after WW2 and things in the 1960s did not happen. No vietnam war, no cuban missile crissis. Also given how the 1950s feel is what is very prevalent in Fallout it would have been during the 1950s.
Fallout as we all know is what the people of the 1950s thought the future was like and nuked to hell.