Suppose Hitler decides to hold off from war until he's 60, in 1949? Meanwhile the German technicians get to work developing the German secret weapons program, which is fully up and running by the late 1940's. A much longer war would result, for one thing. Also, no FDR, Churchill past his best (75 in 1949) and Joseph Stalin dying on schedule in March 1953, puts the war in a totally different light if fought a decade later.