NEW YORK — The U.S. labor-force participation rate for individuals 55 and older fell to 36.9 percent in July, down from 37.9 percent in December. Economists attribute the drop to rising investment wealth enabling older Americans to exit the workforce ahead of schedule.

The dynamic is straightforward: strong portfolio gains make continued employment less necessary. That wealth effect is tightening the U.S. labor supply at precisely the moment demographic aging is already reducing the available workforce.

Most of these retiring workers are heavily allocated to S&P 500 mutual funds and ETFs — vehicles that have delivered a decade of strong returns and pushed account balances to levels that, for many, clear the threshold for retirement.

Generation X — those born between 1965 and 1980 — faces a steeper climb than headlines typically acknowledge. Only 14 percent of Gen X workers hold a traditional pension, compared with 56 percent of Baby Boomers, according to research from Allianz's Retirement Income Institute. The institute's research concludes Gen Xers are the least financially prepared generation for retirement across most measures.

The risk hiding inside that picture is sequence-of-returns. Certified financial planner Ernie Cave, founder of Cave Wealth Management, warns that while markets recover over time, retirees cannot choose when that recovery arrives.

History makes the point with force. Investors who bought Amazon at its 1999 peak waited a full decade for the stock to reclaim that level, finally breaking through in late 2009. The S&P 500 traced a similar path: it bottomed in October 2002 after the dot-com bust, clawed back to a new high in 2007, then surrendered those gains to the financial crisis. From its March 2009 low, the index took another four years to clear the 2007 peak for good, in March 2013.

Those recovery windows — ranging from four to 13 years depending on the starting point — are not academic for anyone three to five years from retirement. Forced selling of depressed assets to cover living expenses locks in losses that compound the damage.

The current wave of early retirements, if sustained, will pressure corporate hiring, push wages higher and constrain overall economic output. The S&P 500 last traded at 7,753, off 0.1 percent on the day; the Nasdaq sits at 26,605, down 0.3 percent.

The market is giving some older Americans the financial confidence to retire now — and creating the conditions for a labor shortage that will cost businesses later.