This from a report [large pdf] published in 1990 by Larry Summers, David Cutler, James Poterba, and Loise Sheiner:
“For about 15 years, the United States runs current account deficits, so that more than 6 percent of U.S. assets are owned by foreigners in 2010. High saving for the subsequent 15 years results in current account surpluses and reduces foreign capital ownership to 3.5 percent. Past 2020, however, with the rapid increase in the number of elderly, the United States again runs current account deficits, so that in the steady state almost 9 percent of U.S. assets are owned by foreigners.”
Well that explains much of the past 20 years, doesn't it? Via A Fistful of Euros, who adds:
So, if we don’t do something, and do something now, to stop median ages rising too rapidly, then more crises are guaranteed, and the next round will make this crisis will seem like, now how do they put it, oh yes, a picnic.