Indeed, a 2015 study by the Transamerica Center for Retirement Studies found that 44 percent of those who retired later than planned said they continued to work because they chose to. Why? "They know they're doing a good job," Dr. Carstensen said.
Case in point: Louis DeHaas, a former city prosecutor in Los Angeles who is now a medical malpractice lawyer. At age 75, Mr. DeHaas, who goes by Duke, estimates that he is in trial about six to eight months a year — a heavy load. Although he is usually the oldest person in the courtroom, Mr. DeHaas said, he's thriving at this point in this career.
"I think I'm a much better lawyer now," said Mr. DeHaas, who lives in Malibu, Calif. "I'm more patient, I understand people better."
He still works hard at it, forcing himself to keep current on music, sports and fashion trends. "I don't want to be judged as an old fogey standing there in a trial," he said, admitting that his 15-year-old granddaughter is his source of insight into popular culture.
Rae Ferguson is also flourishing, at age 71 — although, like Ms. Leonard, in her second career.
Ms. Ferguson is an associate professor of African-American history at the University of Rhode Island. "I'm a latecomer to this profession," said Ms. Ferguson, who was in her 50s when she completed her dissertation at Indiana University — about the role of black women's clubs in Indianapolis in the early 20th century.
Ms. Ferguson, who grew up in Germany, taught music, practices meditation and yoga and has survived cancer, said she believes that her varied life experiences have helped her in the college classroom. "I think I have a lot more ways to hook students in than I would have when I was 35," she said.
Of course, working past traditional retirement age is neither desirable nor feasible for some.
"My basic sense is that there are two types of workers in their late 50s," said Michael D. Hurd, an economist and director of the RAND Corporation Center for the Study of Aging in Santa Monica, Calif. "People who have health problems or are economically distressed, and who might want to work longer but probably won't be able to, will leave the labor force at 62 when eligible for Social Security. More healthy ones will want to work longer for the financial reward. The other is a group who are healthy, fit, able to work and find their work satisfying. They will continue to work into their 60s or even beyond."
Russ Umphenour tried retirement in his 60s. It didn't work. "For a couple years, my wife and I traveled a lot," said Mr. Umphenour, now 72. "It was fun, but I got bored and felt my life had no purpose."
Now Mr. Umphenour, who has residences in Atlanta and New York City, has returned to the restaurant business he worked in all his life. He is involved in several ventures, principally as chief executive officer for Stevi B's Pizza Buffet, a franchise of 29 restaurants in the Southeast.