Over at Japan Economy Watch, Edward Hugh has posted an excellent essay on the above subject, with links to supporting data and related essays available on the web. In particular, take note of the list of similarities in the situations of the "three most "elderly" societies in terms of median age - Japan, Germany and Italy"...
I think the similarities between Germany, Japan, and Italy on Edward's list are all accurate. It is rather coincidental that the three most elderly societies happen to be the countries that formed the Axis during WWII; but I do think it is merely coincidence. In the cases of Japan and Italy, a major factor in the rapid decrease in fertility has been the rejection/decline of family
norms that had supported fertility. In Italy, the Roman Catholic church's positions on birth control, abortion and divorce had served to facilitate high fertility, but with certain general changes by the Vatican in the 1960's I think served to motivate Italians to feel free to reject Catholic doctrines beyond those that were changed, and therefore they rapidly adopted particularly modern birth control methods. In Japan, somewhat more recently, young women have been rejecting the traditional family system where women married, moved into a house with their husband and inlaws, and did not expect to work. Perhaps that is a blinding flash of the obvious, but I think that the rapidity of the change in attitudes among young women in Japan towards avoiding the burdens of traditional marriage and therefore having children is remarkable. I think it boils down to the idea that the traditional family in Japan has been much more burdensome on the wife than in other countries; particularly the USA and Germany, and so given the opportunity Japanese women have been rejecting the traditions wholesale because they don't want to get trapped into a traditional system.
In Germany, of course, Catholicism has not had as strong of an influence as in Italy. It has been more of a case of West Germany reaching US levels of prosperity more rapidly than Japan or Italy, and therefore young people bought into postponing childbirth for economic reasons.
I think a danger to these three countries as great as that of low fertility is their resistance to in-migration of individuals of differing ethnic backgrounds due to their national identities and political systems being based on ethnicity rather than on a set of principles (as opposed to the USA, UK and France which naturalize immigrants of any race). I suppose this attitude might be somewhat less prevalent in Italy due to its central Mediterranean location and historical political turbulence. In the case of Japan, rapid shrinkage of the population would likely make that country even more dependent on the USA for its military defense, the consequences of which are hard to foresee.