I read this sci-fi story at least 15 years ago. The story is about a far future utopia. There were two segments of people, technologically advanced people living in stations or ships, and not so advanced ones living on planets. The station/ship people were living a life of 1000 years, the reproduction is completely controlled and in vitro, the concepts of family, parents, siblings and such are long lost their meaning.
The question thus far is extremely light on plot details, but the dichotomy between long-lived space-dwellers and short-lived (and technologically less-advanced) planet dwellers makes me think of Between the Strokes of Night (1985) by Charles Sheffield.
It fits with the "hard SF" tag because there is no FTL in the universe; the space-dwellers' long life-spans is the only thing that enables a society to exist given the decades-long (or longer) time spans required for interstellar travel.
The novel opens with the initial expansion off of Earth and the investigation of hibernation as a means of surviving long space journeys. This leads to the discovery of a stable metabolic state that allows the perception of time at an extremely slow rate, and a concomitant extension of lifespan.
The story then skips thousands of years into the future and to a planetary society where high-scoring individuals are recruited into the space-dwelling society.
You can read more details in the plot synopsis on Wikipedia.