Pre-1914 Male Population Aged 15-24 in Selected Countries
These figures are based on the censuses carried out by these countries in 1910 and 1911, and I have focused on the male population for the 15-19 and 20-24 age cohorts because those were the primary age groups that would see service in the war. I have not included data for Imperial Russia simply because there is no accurate government data available for the immediate pre-war years. The only census was carried out in Russia in 1897.
Sources: B. R. Mitchell, International Historical Statistics: Europe, 1750-2005, 6th ed. (New York: Palgrave, 2007) and U.S. Bureau of the Census, Historical Statistics of the United States, Colonial Times to 1970 (Washington: USGPO, 1975)
Percentage of Pre-1914 Male Population Aged 15-24 that were casualties
I was interested to make this calculation, even though there are some flaws with this. Using the pre-war age cohort population of young men in these countries, I want to get a rough idea of the casualties suffered by this age group, and since this age group suffered high casualties (See French Losses per Annual Mobilization Class.), I used total casualties for these countries--yes, not total casualties for this age cohort--and calculated the percentage (total casualties/size of age cohort). This gives a rough idea of the impact of the war on young men in these countries.
Sources: See above and Encyclopedia Britannica.