Of course when that happens there will be an oppositional push to have children. I think I saw something about that on TV involving Mongolia, where unlike China they were having a low birth rate. So the culture then turned to celebrating childbirth and encouraging large families, etc.

Also I hear in Japan less women now want kids (probably for good reason.) Perhaps the same may happen there, though...

In a world of decreasing oil, the newly oil-thirsty nations are going to want access to the remaining reserves and may well be prepared to fight to get it.

if you're going to war, how handy it is for to have a vast surplus of young men of fighting age.