young officer; youth officer
Refers to a young military officer, typically in a historical or formal context. The term is somewhat dated and may appear in historical or military literature.
He was appointed as a young officer at an early age.
青年将校 is a more common modern term for a young officer, while 少壮士官 carries a slightly more formal or historical nuance.
Compound of 少壮 (young and vigorous) and 士官 (military officer). The exact historical derivation is uncertain, but the term is conventionally associated with young commissioned officers.