Study leader Michael Lovaglia, a sociologist in the University of Iowa College of Liberal Arts and Sciences, said research indicates Americans expect women to reach their peak performance as leaders at age 43 -- four years before men's perceived peak at age 47.

The nationally representative online survey of 1,996 U.S. adults found most thought women's contributions at work start to decline at 59.7, compared to age 61.3 for men.

These leadership age expectations may have hurt Clinton, who is 60, but helped Sen. Barack Obama, D-Ill., who will soon hit the "ideal" leadership age of 47, Lovaglia said.

The research does not bode well for Sen. John McCain, R-Ariz., if many believe men's contributions at work begin to decline at age 61.3, McCain is 71.

"He's going to have to try very hard to manage that issue and overcome people's expectations that he's lost a step," Lovaglia said.

Lovaglia said the study could also explain the surge in the popularity of plastic surgery.