Teens and adults don't use the same mesh for their bodies. In order to use outfits built on the adult mesh, your teen will also have to "wear" the adult body.

I recently decided to use adult clothing for teens and i'm aware of the issue of the neck and stuff but when i actually confirm everything in CAS,when i go back in game it's like the game doesnt read adult clothings on teens so the sims automatically replace it with actual teens clothes,do you guys know what i have to do to fix it?PLEASE IT'S URGENT!

Unfortunately EA broke the cross age features of MasterController and I don't have a solution for fixing them at this time. Also please make a new thread next time.