I don't think you can find an easy source to collect the ages of all French deaths, but some simple math may shed some light. Out of a nation of 39+ million, slightly less than 1.4 million deaths recorded from combat, wounds, disease and missing presumed dead. That would presume that there was at only no more than 2.7 million males between 20-30 in a nation of 39 million.
The starker and more realistic number is that 8.4 million mobilized and 6.1 casualties including dead, wounded and captured. It is likely that your source may have implied that 1/2 of that generation bore some wound or indignity.
I don't have age-based demographic data, but "La Vie en Bleu" by Rod Kedward offers some other unique demographic calculations of the French in WWI: "Number had been a major theme of the war...The national statistics gradually revealed that over 1,400,000 had been killed in battle, were missing or had died after the war as a result of injuries, including 81,000 of the 600,000 indigenous troops. [from colonial possesions] (pg. 90) "The demographic calculations also included the deficit in birhts over the four years of the war, estimated at a futher 1,400,000, those killed on the home front, and the 200,000 who died of the influenza epidemic of 1918. In total the population loss of the war years reached 3 million, only partially offest by the 1.9 million in Alsace-Lorraine who returned to French citizenship. Two-thirds of those killed at the front were from rural occupations. Peasent communities were decimated." (p. 91) "In France as a whole one in every six men enlisted was killed, the toll rising to over a quarter of those sent to the front." (p. 91) "Agricultural work accounted for roughtly 43 per cent of the working population in 1906: three-fifths of all men in this secctor were mobilized, totaling 3.5 million. Almost all served in the infantry." (p. 92) So while that's not your 20-30 year old sample you wanted, I thought the 20-30 age group and the agricultural/rural working population would have significant overlap based on Kedward and Weber's "Peasents into Frenchman." Hopefully it satisfies a little of your curiosity.