After various testing... seems like it's a bug that teens get stuck in the Loud Phase (or any phase actually) for more than 3 or 4 sim days. I've had this teen sim in the loud phase for now 14 sim weeks as I have aging off and despite his parent being at L10 of the parenting skill there is really nothing that works. So best bet is to cheat the trait away then save and quit your game and delete the localthumbcache.package file.
testingcheats on followed by:
traits.remove_trait ChildhoodPhase_Loud
If your sim still displays the symptoms of being in the Loud Phase even after applying this cheat etc just take him/her through a few loading screens.