America is remarkably fertile for a rich country. Its women produce 2.12 babies apiece, on average. Korea and Japan fare far better in car production than reproduction, with fertility rates of 1.19 and 1.34, respectively. Germany, Italy and Spain are running short on new Maximilians, Alessias and Alejandros, with rates below 1.5. The U.K. is relatively blessed with a fertility rate of 1.96 and France spends mightily on family perks to achieve a 2.02. But all rich countries except for the U.S. and Ireland fall short of the critical replacement level of 2.1, which decides whether a population grows or shrinks, absent immigration and changes to mortality.

The world is growing old. Since around 1870, citizens of developed countries have gradually lived longer and bred less. The median age for all nations has climbed to 29 from 25 since 1950. It will jump to 38 by 2050, according to the United Nations, and it will be held that low only by poor countries, which contribute 95% of the world’s population growth. Germany will reach a median age of 52 by 2050. Japan will be 55 and Italy 51. France will look relatively sprite at 45, but America, the youngster of rich nations, will not yet be 42.

It’s not especially clear why American women are keener on motherhood than their counterparts in other countries. One theory credits America’s fairly flexible labor force, which allows women to leave and re-enter jobs with relative (if not absolute) ease. Another theory holds that Americans are generally supportive of working moms, which makes procreation more affordable. Also, note that birth rates in general tend to rise during periods of strong economic growth and fall during downturns, so early numbers for 2009 suggest U.S. fertility is down a touch—but the lead over peer nations is still wide.

There’s more at stake here than a steady supply of toothless smiles and strong demand for diapers. For nations, median ages are crucial in deciding the long-term affordability of social programs. Whatever the changes to America’s health-care and public pension programs in coming years, the young will surely pay for the old, as they do around the world. More working-age citizens within a population means a lighter burden for each, all else held equal.

I point this out as further evidence that America is financially troubled but not nearly doomed. Against a basket of six major currencies including the euro and yen, the dollar fell 4.3% in the third quarter after a 6.2% decline in the second quarter, seemingly on fears that the government will substantially dilute the currency to pay its mounting debt. I recently argued here that selling in the U.S. dollar seems overdone. America’s debt is growing far too quickly, and much of the money is being spent on folly, like cash incentives to buy houses and cars, which perversely tempt consumers who have overspent on just such items in recent decades to spend more. But America’s debt held by the public is still relatively affordable at 49% of its gross domestic product, vs. 54% for Spain, 65% for the U.K., 75% for France and Germany and 186% for Japan.

Don’t read this as a rally call for a particular investment class. Stock prices still look too high to me. Houses are still a touch too expensive in most markets. Gold isn’t the safe haven it’s made out to be. Dollars seem cheap, so savings accounts look like a better deal than their paltry yields suggest. But quibbling over valuations aside, the U.S. remains an excellent place to invest for coming decades. The economic fashion of the moment is to say the country is past its prime. An army of tiny, gurgling workers-to-be says otherwise.