Wesley Brown, Agnes Zhelesnik and Seth Glickenhaus are older than 95 and still working; not because they have to but because they want to.
"I love my work," Glickenhaus, who is 97 and a senior partner at investment firm Glickenhaus and Co. in New York City, said. "I'm the Tiger Woods or Michael Jordan of money and investing."
Glickenhaus started working in 1929 during his summer breaks from Harvard and has been working ever since.
"I plan to stick around for many decades," he said. "I'm just approaching middle age."
Dr. Thomas Perls, director of the New England Centenarian Study at Boston University, said it all comes down to an individual's cognitive ability.
"As long as they are cognitively intact, they are virtually immortal," he said. "It is only once we see a decline in their cognitive function is their mortality quite high."
At 103, U.S. District Judge Wesley Brown is the oldest federal judge in the nation and has taken his lifetime appointment from President Kennedy to heart.
"As a federal judge, I was appointed for life or good behavior, whichever I lose first," Brown, who presides over the court in Wichita, Kan., said. "I will quit this job when I think it is time. ... And I hope I do so and leave the country in better shape because I have been a part of it."
Age 90 in Reach of Most People
"Most of us have the genetic blueprint and environmental advantages to live to about age 90," Perls said. "It requires a very healthy lifestyle, basically the older you get, the healthier you've been.
Judge Brown said, "You got to have a reason to live. As long as you perform a public service, you have a reason to live."
While Brown lives for service and Glickenhaus lives for finance, Agnes Zhelesnik lives to teach. At 97, Zhelesnik is believed to be the nation's oldest full-time teacher.
Zhelesnik teaches cooking and sewing at the Sundance School in North Plainfield, N.J.
"Old people, when they have nothing to do, they stay in the room by themselves," Zhelesnik said. "It's not easy getting old.
"I just look forward to cooking with the children. The children is the principal thing. ... As long as I am able to do things and Sundance accepts me ... then I'll stay here."
Investment adviser Glickenhaus agrees and says that being eager to retire is not a good thing.
"The only bit of advice I would give the world is get a job that you enjoy doing; it gives you a lot of satisfaction," he said. "There is nothing sadder than hearing someone say, 'I'm going to retire next year, thank God.' It speaks to a misspent life."
Such a sense of purpose and meaning plays an important role in longevity, Perls of Boston University said. "It is vital to have a cause to wake up to every morning," he said. "There has to be something to get you up and keep you engaged."
Neuroticism Seems to Shave Years
Perls said people like Brown, Zhelesnik and Glickenhaus tend to be extroverts who easily establish friendships and thrive on their relationships with others. They also usually handle stress well and score low on neuroticism.
The secret might just be in not looking too far ahead.
"I celebrate everyday, I've done that all my life," Glickenhaus said. "I don't really believe much in birthday celebrations. When I'm 125, I might think about celebrating. ... I take it very much in stride."
ABC News' Linsey Davis and the Associated Press contributed to this report.