I read this a long time ago so it is at least 20 years old but quite probably older. I'm rather foggy on the details.
I believe it was either a novella or novel and had distinct chapters on different crew candidates.
The bulk of the story is about a training program where a fairly large group is trained together to crew an enormous spacecraft that is being built concurrently.
The group is very talented but only the best (6?) will actually make it to the spacecraft. The story is about one class in a series where every (4?) years or so a new launch occurs. The final selection and launch will happen at the youngest feasible age to allow as much time as possible for the main goal: Find new habitable planets.
The story ends shortly after launch and leaves the crew in dire straits, as the course set deviates for some reason and they will end up in unknown space far outside the range of the (few?) existing colonies.
I recall that one candidate is genetically enhanced and because of this has trouble integrating with the other trainees.
I'd love to read it once again. I'm also curious whether there is a sequel. Or a 5-part trilogy or some such.