They have a shortish (10-15 year) window to do anything serious - as the population ages they will have to spend more and more on health care etc (sound familiar?) and they don't have a load of immigrants ready to arrive
the current 1-5 and 6-10 year cohorts are around 75 million each - but the 45-50 group is about 145 million = in 10 years every young worker will be supporting two of the elderly
Less and less money will be available for the military