Economists point to a long list of reasons, a complex mix of pushes and pulls.

Retirement policies have shifted: Fewer employees have fixed pensions, which tend to move people into retirement because at some point, they’ve earned maximum benefits and more years of work don’t bring more income.

“In contrast, there’s no such age with 401(k)’s,” said Alicia Munnell, the director of the Center for Retirement Research at Boston College. “People are nervous about that. They want their pile to grow, so they stay in the work force longer.” An increase in the Social Security full retirement age has had some effect, too.

We’re also working longer because we can. True, health crises can strike at any point, but lengthening life spans and improved health, at least among higher-income seniors, have contributed to extended careers.

“People who have higher education levels work longer,” Dr. Munnell said. “They have nicer jobs that are more interesting and less physically demanding.”

Older workers are more apt than others to work in management and sales, the Pew report found, and less likely to work in construction or food preparation and service.

Economists applaud this graying work force. “Every additional year you work, and you don’t draw down your savings, is an asset,” said Debra Whitman, the chief public policy officer at AARP. Waiting until later ages to claim Social Security — as seniors increasingly do — might help keep the system solvent, and the higher monthly checks may keep households solvent, too.

(AARP calculates that a 66-year-old entitled to a $1,500 monthly benefit at her full retirement age would instead be receiving only $1,125 — for the rest of her life — if she opted for early retirement at 62. If she can delay claiming benefits until she’s 70, though few retirees wait that long, her lifelong monthly income would grow to $1,980.)