Period: 1960 - 1990
Main character: a man
Clue: The main clue is that a group of humans live on the South Pole, called "The Girls". The interesting thing about them is that they live to a very old age or are immortal, because they choose to not age beyond puberty. So, basically, they're all prepubescent, they never menstruated, but at the same time they are very, very old.
However, they are not the main characters, they're there to help the story along because they have some knowledge.
The rest of humanity is either scattered or doesn't exist anymore, I forgot what happened to them, but it's a post apo novel, so ....
Period: 1960 - 1990
I believe this is "A World Out of Time" by Larry Niven.
From the Wikipedia description:
The Earth's climate has changed, despite its new location in orbit around Jupiter. Among the most important changes is the increased surface temperature; the poles are temperate, while the former temperate zones reach temperatures of over 50 degrees Celsius (120+ degrees Fahrenheit). The Earth's axial tilt is still 23.5 degrees so the poles experience 6 years of night and 6 years of day. Almost all remaining life on Earth has adapted to live in Antarctica. Elsewhere life is extinct except for some evidence of biological activity in the Himalayan mountains.
When Corbell lands (in a modified biological probe), he is captured by Mirelly-Lyra, who is also a returned star ship pilot and refugee from the past—though from Corbell's (and Peerssa's) future. She explains that the human species has fragmented; it is dominated by a race of immortal, permanently pre-adolescent males (the Boys), who are created by advanced medical techniques. Some time in the past, they had defeated the equally immortal (though now extinct) Girls, in the ultimate war of the sexes. The Boys have enslaved the dikta, who are unmodified humans (though they have evolved somewhat), from whom they take boys to replenish their ranks.
A starship pilot, Mirelly-Lyra had initially been a captive toy of the Girls. After their downfall, she had spent her time obsessively searching in vain for the lost adult-immortality treatment, while extending her life as much as possible using her own drugs and a form of zero-time stasis with which she waited for another returning star ship and potential help. Because she could not stop the aging process entirely, she is an old crone by the time she finds Corbell. He manages to escape from her, only to be caught by the Boys, who take him to a dikta settlement. Corbell finds out that the solar system was engineered into its new configuration by the Girls in order to move the Earth to a habitable distance from the enlarged Sun (caused by war with colonies), and that an orbital error caused Jupiter to overheat and triggered the war that killed the Girls.