• Malaysia's former prime minister Mahathir Mohamad, 99, says eating in moderation is the key to longevity.
• "Age does not follow time. Age follows bodily health — the way you can function," he told Nikkei Asia.
• The number of people aged 80 or older is expected to reach 426 million worldwide, per WHO estimates.

Malaysia's former prime minister, Mahathir Mohamad, will be 100 next year. His secret to longevity? Moderation.

"I don't eat too much. Obesity isn't good for survival," Mahathir told Nikkei Asia during an interview published in November.

"Age does not follow time. Age follows bodily health — the way you can function. Because of that, old age is not because of time; it's not because you are 60 that you are old. You may be 90, but if people at 90 are still strong, still able to work and function, then 90 would not be old age," he said.

Mahathir added that what people think of as old age has changed over time.

"At this moment, 60 or 65 is considered old, but we must remember that in the past, 30 was already old. During the time of Julius Caesar, leaders were only in their 30s because they died very early," he said.

Mahathir first served as Malaysia's prime minister from 1981 to 2003.

He reentered national politics in 2018 in a bid to unseat former prime minister Najib Razak, who had been accused of embezzling millions in state funds in the 1MDB scandal.

That same year, at age 92, Mahathir was elected as the country's prime minister once more — although he resigned a little under two years later.

Looking back on his political career, Mahathir shared that he regretted resigning from his post in 2003, "when I was not even 80 years old."

"When I was in my 70s, approaching 80, I told myself that 80 was very old. I thought I wouldn't be able to function well anymore. However, after I retired, I found that I was still able to work, still able to function," Mahathir said.

Experience comes with age, and society should try to retain older workers so as to not lose the wealth of knowledge that this demographic can provide, he said.

"If you retain older people in the workforce and allow them to continue working instead of retiring too early, you can benefit from their experience," Mahathir said.

During his second tenure as Malaysia's prime minister, Mahathir was considered one of the oldest elected world leaders. The oldest current national leader is President Paul Biya of Cameroon, who is 91 years old. In comparison, Joe Biden, at 82, is the oldest American president.

The number of people aged 80 or older is expected to triple between 2020 and 2050 to reach 426 million worldwide, per WHO estimates.

A representative for Mahathir did not immediately respond to a request for comment sent by Business Insider.