T. Boone Pickens, the iconic oil and natural gas investor, is getting out of the business he helped shape since the 1950s.

Pickens, 89, chairman of BP Capital Fund Advisors, announced in a letter that he is shutting the commodity and energy stock hedge funds he runs. He cited poor health and advanced age as the reason.

"It's no secret the past year has not been good to me, from a health perspective or a financial one," he wrote in a letter he posted on LinkedIn on Friday. "Health-wise, I'm still recovering from a series of strokes I suffered late last year, and a major fall over the summer. If you are lucky enough to make it to 89 years of age like I have, those things tend to put life in perspective. It's time to start making new plans and setting new priorities."

Pickens said it's the third time in his career that he's walked away from a job. He said he quit a job as a geologist with Phillips Petroleum in 1953 at 25. He said the bureaucracy of a major oil company was frustrating. He soon started Mesa Petroleum, a company he ran for nearly 40 years until he retired in 1996 at 68.

"In both instances, I had a plan, and my life was much richer as a result," he said. He said he became a billionaire for the first time after that 1996 retirement.

During his time running Mesa he became famous as a corporate raider, making bids for much larger oil companies such as Gulf Oil, Phillips Petroleum, and Unocal. He appeared on the cover of Time magazine in 1985 holding playing cards decorated with oil wells, under the headline "The takeover game." While he didn't end up buying any of those companies, his offers often forced his targets to acquisitions by other companies, which produced large profits for Mesa and his shareholders.

While he is most famous as an investor in oil and natural gas, he came out with a proposal in 2008 to shift away from oil through increased use of wind power and a shift to natural gas for cars and trucks. But the Great Recession soon ate into oil demand and sent gas prices plunging from the record high price of that summer.

Pickens said it is important from him to step away from his business interests so he can get his affairs in order and devote his full attention to recovering his health.

Related: Even T. Boone Pickens doesn't see oil hitting $100 any time soon

He wrote that during his career he's seen oil go from $10 a barrel up to $147 in 2007, and then back down to $26 during the depths of the Great Recession.

"I'm ecstatic that I've hung on long enough to see it all unfold. I've thrived and profited on the volatility in the energy space," he wrote. But he said that despite the recent rally in energy prices, "for me, personally, trading oil is not as intriguing to me as it once was."