T. Boone Pickens, a famous oilman and investment manager, said he is closing the energy-focused hedge fund he has run for the last two decades as his health declines.

The move closes a chapter in a nearly seven-decade career that has included stints as a wildcatter, corporate raider, cattle trader and clean-energy evangelist.

The 89-year-old Oklahoma native cited both his deteriorating health and weak financial performance as reasons for closing the fund in a letter that he published Friday. Mr. Pickens said he is recovering from a series of strokes last year and a bad fall.

“It’s no secret the past year has not been good to me, from a health perspective or a financial one,” he wrote in the letter. “If you are lucky enough to make it to 89 years of age like I have, those things tend to put life in perspective.”

“Trading oil is not as intriguing to me as it once was,” he added.