Space Cadet, the novel, was released in 1948. I haven't read it, but it's pretty easy to guess at the plot:
- Someone from a small town enroles in some form of Space Academy
- We follow them as they struggle to graduate from the academy
- Disaster strikes and is averted
- We see them on to their first mission as an officer
- Disaster strikes again, and is averted, again.
Importantly, though, the story was written for young adults, and features a YA protag.