That's not naoto's hang up though. Her insecurities were more about being seen as young and inexperienced to the actual police force, not the investigation team, plus also being female in a male dominated work force.
Also if he's part of the crew wouldn't be still be in high school?
He appears older than the crew. It's leading a lot of folks to speculate he's a young adult. Most common theory is he's fresh out of the police academy.
A young police officer realizing the world he's chosen to go into is corrupt and broken would fit into what seems to be a lot of the major themes of P5 as well. It wold also be a much different arc from Naoto. Dealing with potentially destroying your career to pursue the greater good is a potently compelling character arc.