Immersion of what exactly?
The game has a very specific timeframe. None of the people born during the game have any chance whatsoever of getting involved in the war, they would all be young children. In fact, you can pretty much extend that to the entire 30s. Seeing how the overall population is completely irrelevant in every way, there is little reason to spend time on something like that. It has no impact on manpower or production, or anything else for that matter. The slight increase to the people who are available per draft-class that some nations may have had - which is connected to the birth-rates of the late 10s and entire 20s - gets incorporated through the manpower.
There are plenty of issues that are far more important than that. There are also things that are far more connected to the war that won't make it into the game, so if it comes to adding immersion those things should be way ahead of something that does nothing for the game.