Eric Schmidt said working with 98-year-old Henry Kissinger convinced him that the secret to longevity is being a workaholic

• Eric Schmidt said that he is convinced that the secret to a long life and career is 'being a workaholic'.

• Schmidt, a former Google CEO, has co-authored a book with 98-year-old Henry Kissinger.

Former Google CEO Eric Schmidt said that he is convinced that the secret to a long life and career is being a "workaholic" - something that working with Henry Kissinger had taught him.

Schmidt appeared on the Tim Ferriss Podcast to talk about his new book 'The Age of AI' which he co-authored with the 98-year-old former US Secretary of State and Dan Huttenlocher, dean of computer science at MIT.

Podcast host Ferriss, who writes about productivity and life hacks, asked Schmidt to explain Kissinger's ability to remain cognitively sharp late into his nineties.

"He works harder than a 40 year old," said Schmidt. "I can tell you that he gets up in the morning and he works all day. He has dinner with his wife and his family and he works at night."

Schmidt added that at the age of 90, Kissinger knew nothing about the digital world, but mastered the the subject with the "alacrity and speed of people who are just getting into it now."

"I am convinced that the secret to longevity is being a workaholic," he continued.

Schmidt said he first met Kissinger at a conference, at which the former diplomat told him that he was worried Google was "going to destroy the world."

Schmidt then invited Kissinger to talk to Google staff and said that the two struck up a friendship despite their differing political beliefs. They started collaborating on the book with Huttenlocher at Kissinger's suggestion, said Schmidt.

Kissinger is a divisive figure in US political history, thanks to his role as Secretary of State under President Richard Nixon and under Gerald Ford. Some see him as a master diplomat who played a crucial role in ending the Vietnam war. Others call him a war criminal.

Since leaving government he has written several books, sat on company boards - including failed blood-testing startup Theranos - and gives regular speeches.

Schmidt's comments are specific to Kissinger's enjoyment of his work, and he didn't go into detail about the former politician's daily work routine. While a few people are able to balance long working hours, academics warn that workaholism can lead to social isolation, burnout, and depression.

Psychologists say there are many potential reasons determining a person's long term success.

Angela Duckworth argues in her New York Times bestselling book 'Grit' that passion and perseverance are the key to why 'gritty' people stick to long term projects.

Focus and constant development are other traits regularly highlighted. Physical health and wellbeing is also crucial.

Berkshire Hathaway founder Warren Buffett and his right-hand man Charlie Munger are other high profile nonagenarians. Luck, choosing bets wisely, and maintaining a virtuous circle are some of the reasons Munger attributes to their continued success.