Demographics is destiny; this is what happens when you don't have any children. Look for a repeat perfomance soon in Italy, Spain and Germany...and especially Russia.
There probably are not going to be any more wars like WW-II with 3000 mile fronts, nor any real need for countries like Germany or Russia with the kinds of weapons they have to need 5,000,000 people carrying rifles at one time. There is no real reason why a country with 100,000,000 people needs half of those people to be men. If you were to artificially skew sex ratios to something like 1 - 6 or 1 - 8 instead of the natural (for humans) 1 - 1, then a birth rate of 1.3 such as Germany has had recently would be more than enough to keep population levels together.