What's referred to as the ADK study of Soviet demographics (From authors Andreev, Darskii, and Khar'kova) in Население Советского Союза: 1922-1991 gives a death toll of 9.6% of males aged 15-19 in 1941 and 36% of males aged 20-34. You're not gonna get perfect numbers, but that gives a fairly good idea of the demographic breakdown.

To give a relevant comparison, France's 20-23 class of men in 1914 when WW1 broke out suffered 31% mortality of those enlisted and 22% of the entire cohort (Héran, "Lost generations: The demographic impact of the Great War"). The USSR's deaths among military aged men were on the extreme end of mass mobilized societies in WW1-WW2, but they weren't incomparable to those of other states. Modern warfare was exceptionally deadly for the young men and women involved. What really makes the USSR's death toll stand out is the massive number of civilians killed, which far exceeds the death toll from combat.

So I wouldn't call those numbers a "myth", just hyperbole. Without a comparative element you can't discern which parts of the Soviet-Axis war were extreme (Mass mobilization and deaths among young men) and which were exceptional (Mass genocide of POWs and noncombatants).