The protagonist is the youngest person on Earth -- the only one to be born in many centuries. Terrestrial society is small but every individual is unique and multitalented, as they are all immortal and have lived for centuries. They live in flying habitats.
No one knows how to raise children anymore; the protagonist wears diapers into adulthood because he was never potty-trained, and his education was haphazard. Consequently, he is quite lost in this advanced society, and only tolerated by the sophisticates around him.
When the Earth is invaded, the immortals put up only a show of a fight, arriving at the scene of battle in a colorful flotilla of unarmed flying habitats. To the horror of the protagonist, they are wiped out by the invaders. He realizes that the immortals were weary of life and welcomed this end.
This is probably a novel, though it might be a novella. It was written in English and I read it probably in the 1970s or 1980s.