Prince Philip remained a working member of the royal family until the age of 96. While it's certainly not mandatory (or even recommended) for people to work well past retirement age, studies have shown that it may improve one's chances at maintaining good health and keeping severe health problems at bay. Staying active, social and busy for so long may also have helped keep Prince Philip's mind sharp and increased his overall happiness and fulfillment. Having a sense of purpose has been seen as a common factor of longevity among peoples who live exceptionally long lives. The important thing is to love what you do!