POP growth does not rely on gender ratios, but it should.So there will always be remaining pops, even if all of your levies get stackwhiped or otherwise the game would be broken. If those surviving pops are surviving male pops or mostly women, children and old people is irrelevant from the gameplay perspective, as pop growth don't rely on gender ratios.
If you lose all your young men in battles, the rate of reproduction of your POPs should decrease accordingly. I am more in favor of this solution instead of killing POPs after one horrible battle with a lot of dead males. But then we should look at the population growth factors.