Dude, their behavior is Saiyan-based, not age-based.
What's next? "Adults aren't that caught up in pride, so Vegeta must be 10 or Goku is such a retard and pure, he must be 5....
You want them to be underaged.
Also, if they are of the same age as Cabba, who is in the army(!), they can't be underaged.