I think it was a chapter in Malcolm Gladwell Freakenomics where he pointed out the birthdates of pro level hockey players in these types of countries. Birthdates were always heavily stacked to January. It was concluded that the skills development at the young ages of children made a huge difference in who was selected for more advanced training. When the kids started out, they were put in age groups. A 5 year old born in Jan has a HUGE developmental advantage over a 5 year old born in Dec. when comparing them for athletics.
It’s not just the communist countries that are like that. Canada was notorious for that age/month factor you describe there. A disproportionate number of top NHL players over the last 40+ years were born early in the year, including Wayne Gretzky (January), Mark Messier (January), Gordie Howe (March), and Bobby Orr (March).