I addressed part of this question a while back in an answer:

Do we have any statistics on the life expectancy of a Russian male born in 1923, who would have been 18 when the Great Patriotic War began? What about a German male born the same year?

. . . more can always be said about a topic, but you might start there. I think 1923 is taken as the starting point because that's someone who would have been 18 in 1941 -- eg someone who would have had the most years in which he could have served, someone born in 1926 is only 15 and therefore presumably exposed to a smaller risk of combat fatality.

As the data show, though-- the risk of death prior to 1941 was substantial for this cohort anyway, so combat exposure is somewhat less important as a source of excess mortality than you might otherwise imagine.

Population data do not suggest that births differed dramatically in the 1920s, but data is patchy. The most notable year to year effect in the early 1920s is the 1921-22 Povolzhye famine; may have killed 5 million people, and depressed birth rates and impaired the health of those who were born.

N.B -- this is different from the famine of 1932-33, known in the Ukraine as the "Holodomor", but nationwide -- which is what people usually are referring to when they speak of "Soviet famine".

NAKAI, KAZUO. “Soviet Agricultural Policies in the Ukraine and the 1921-1922 Famine.” Harvard Ukrainian Studies, vol. 6, no. 1, 1982, pp. 43–61.