In the movie, Valerian and the City of a Thousand Planets (2017), Valerian has the rank of major and Laureline is a sergeant. Yet, they seem so young. Is it just that the intended audience is younger, so younger actors were chosen to identify with?
If the ranking system is the same in the future, major and sergeant aren't that high up. And age isn't really a factor so much as your ability to handle the privileges and responsibilities of the rank.
Clearly Valarian and Lauralene earned their ranks.