I'm not sure what the monthly manpower is supposed to represent. If it represents younger cohorts aging into the conscription range, wouldn't you also have cohorts aging out of the conscription range? I think you would need to model those aging out of the military to balance those getting too old for the workforce. Or is the idea that the popular distribution is pyramid-shaped, so those aging out are always fewer than those aging in?
I completely forgot about people aging out. The healthy population pyramid is narrow at the top because of deaths. Those deaths occur typically well after service age has been passed. To model that more accurately the country would need a base population from which to conscript a relatively consistent and stable number of people, which number would be adjustable based on the laws of that country. The base population could grow or shrink but the manpower pool wouldn't be expanding nonstop until the game ends.