What keeps the US economy afloat in these troubled waters? Robert Samuelson says it’s easy credit and aging Boomers:

To some extent the life cycle defeated the economic cycle, Sterne says. Families wanted bigger homes. Their children, flooding high schools and colleges, demanded computers, CDs and cars. From March 2001 to March 2003, says Sterne, several dozen product categories experienced “remarkable” double-digit increases, including cell phones, motorcycles, toys, jewelry and hardware.

One reason Americans could spend freely is that they went deeper into debt. Indeed, the democratization of debt is a great story of the late 20th century. In 1946, just after World War II, consumer debt amounted to 22 percent of household after-tax income, reports the Federal Reserve. (That is, for every $10,000 of income, there was $2,200 of debt.) Now debt is almost 110 percent of income. More families borrow, and debtors have more debt in relation to income.

But like any debt binge, it has to end someday:

The bad news is that all the good news won’t last forever. Spending demographics will deteriorate slightly in the next decade, says Sterne. Younger households — relatively poorer — will grow rapidly. An aging baby boom will slowly lose purchasing power. The larger and iffier issue involves the inevitable, though undetermined, end of America’s 60-year credit binge. Interest rates have risen from recent lows, and greater threats loom.

Household debt can’t permanently grow faster than household income, though it has for decades. Sooner or later families will decide they’ve borrowed enough, or too much. Sooner or later baby boomers will pay down lifetime debts. Sooner or later lenders will exhaust good credit risks. Indeed, whether the aggressive lending of recent years has gone too far remains unsettled. Higher delinquency and personal bankruptcy rates are causes for concern.

Then there’s another factor Samuelson leaves out, one that might be a cause for even greater concern.

The Baby Boomers will start to retire in 2011. And what do retirees do? They begin to sell their equities to pay for their retirements. My generation, the so-called Gen X, has too few people to keep stock prices up at the levels where boomer investments kept them. And Gen Y will still be too young (and therefore poor) to make up the rest of the difference.

The economic boom of the ’90s (and the economy’s resilience during the Current Mess) was more due to a happy confluence of events than it was to any policy from Washington. That’s not to say that our balanced budgets didn’t help, but they were less a cause than they were an effect.

Depressed equities markets, increased Social Security and Medicare payouts, higher interest rates, and tougher credit are all in store for us just a little down the road.

Hope you didn’t get too used to the good times.