I read a sci-fi series where some of the main characters belonged to a society of the oldest people on the planet, where the oldest member was the chairman. Any ideas what it is called or who wrote something like this?
I think that's Methusaleh's Children by Robert Heinlein, though it could conceivably also be Time Enough For Love which uses the same background.
The Howard Families derive from Ira Howard, who became rich in the California Gold Rush, but died young and childless. The trustees of his will carried out his wishes to prolong human life by financially rewarding those with long-lived grandparents, who married each other and had children. By the 22nd Century, these families have a life expectancy of 150 years, and have kept their existence secret, but reveal themselves to the society established under The Covenant, which refuses to believe that the Howard Families obtained their lifespan by selective breeding, instead insisting they have developed a secret method to extend life. Though the Administrator of the planet, Slayton Ford, is convinced the Families are telling the truth, he cannot prevent efforts to force the Howard Families to reveal their (non-existent) rejuvenatory abilities.
To quote from the text:
Mary got out, said hello to the guards and went on through a tunnel into a large underground room where fifty or sixty men and women were seated. She chatted until a clock announced midnight, then she mounted a rostrum and faced them.
"I am," she stated, "one hundred and eighty-three years old. Is there anyone here who is older?"
No one spoke. After a decent wait she went on, "Then in accordance with our customs I declare this meeting opened. Will you choose a moderator?"
Larry Niven's "Tales of the known space" have the Struldbrug club (named after the immortal, but constantly aging people of Luggnagg in Swift's Gulliver's Travels). One prominent character is a member (but not chairman) in some of the stories (namely in Protector), the detective Lucas Garner.
Not a single Struldbrug was less than one hundred and fifty-four years of age, by Club law, and the age requirement went up one year for every two that passed. They needed the best and most rigid of medical protection.
The oldest member is (at least in name) head of the club:
“No. Wait a minute. Estelle Randall? President of the Struldbrugs’ Club until — um.”
“She was my great-to-the-fourth grandmother.”
“And she died last month. My condolences.”
“Thanks. I, I — see. I didn’t see Greatly ’Stelle that often. Maybe twice a year, once at her birthday party, once at a christening or whatever. I remember we had lunch together a few days after I found out I’d lost all my money. She was mad. Oh, boy. She offered to refinance me, but I turned her down.”
“Pride? It could happen to anyone. Lawrence St. John McGee practices an old and polished profession.”
“She was the oldest woman in the world.”
“I know.” The presidency of the Struldbrugs’ Club went to the oldest living member. It was an honorary title; the Acting President usually did the work. “