Methuselah's Children

This is an old story by science fiction standards, originally published in a shorter form in 1941, then expanded in 1958. If you're accustomed to reading old sf, it's readable enough and includes various ideas of some interest, although the plot is highly implausible at various points and characterization is rudimentary.

It features a secret society that is deliberately and successfully breeding humans for longevity, which is quite interesting and a promising start to the story. However, the secret gets out, the society is persecuted by the rest of humanity, and all hundred thousand of its members manage to escape by stealing a huge interstellar spaceship that is very conveniently both present (in Earth orbit) and unguarded. Even more conveniently, one of them promptly invents a new spaceship drive that accelerates the ship to close to the speed of light.

They go on to discover two planets in different solar systems, Earthlike but inhabited by scarily powerful beings of different kinds, before eventually returning to an Earth that is now less hostile to them.

This novel introduces Woodrow Wilson Smith, also known as Lazarus Long, born in 1912 and already 213 years old at the start of the story. He turns up again in various of Heinlein's later novels.