Side Issue On Timelines and Long Term Conflict Mortality
A classic trope in media is the country that is constantly having a revolution, with the logical extreme being a geographical region where any map older than a year is woefully out of date as new nations rise, borders shift, old nations fall and more coups are had than courses at the following inaugural banquets
While maps change rapidly, there is a pretty much inevitable tradeoff between how long the conflict endures and how rapidly things change. If the situation is so dynamic that maps are changing in the region every year or two, the conflict is going to resolve itself in less than a decade or two. If you want a conflict that endures for half a century or a century, for example, while there is going to be significant change from time to time (and even that tends to happen in synchronized waves followed by periods of stagnation and stalemate) the conflict isn't going to progress quite that fast.
However, I want to be able to explain how this region manages to stay so politically active when every settlement in the area would have lost their entire population of 20-40 year olds after about ten years.
While you can have a world like that, there is really no precedent for that level of devastation happening that quickly as a result of war.
Most people who die in this eras aren't battle field casualties but casualties of the breakdown of civilization including war induced famine and disease that make the age breakdown of deaths more spread out. And, there are pretty much no historical military conflicts that killed more people than, for example, the Great Northern War over twenty years (which was a proxy conflict) in Finland in the 18th century that reduced Finland's population by about 50% and was followed by several other major conflicts in the 18th century that reduced the population somewhat further (but again, not primarily from battle deaths of military aged men).
Basically, people surrender no matter what the cost, or become refugees, before an entire generation of men is entirely eliminated, and conflicts that have that depth of impact take more than a decade.
You generally only lose an entire generation of men in historical scenarios of conquest, akin to those described in the Biblical books of Judges and Numbers, or in parts of the New World in the Conquistador era, for example, where an invading Army kills all the men and all of the women unfailingly loyal to them, and then the invading Army takes the remaining existing women as wives replacing the existing men entirely but taking their place, rather than leaving a vacuum.
Also, it is worth noting, that major wars like these generally lead to very high childbirth rates, approaching biological limits, although it is a less strong factor in leading to polygamy than you might naively expect (see, e.g., the case of very high rates of monogamy or non-remarriage among Southern Civil War widows even in communities losing a very large share of their marriage aged men).
Caveat