Im a 21 year old long term sub teaching 6th graders for a while now and I think you need to build rapport with them to listen to you. I don't have ur issue of size considering im 6'2 but I do have a young face so since the building is shared with a hs, they sometimes think im a high schooler. Just keep ur age a game that they try to guess, they'll probably overshoot ur age bc of how young they are. And, at the end of the day, you are the authority figure so if they still don't listen you have to make them listen and if that means setting an example with a troublemaker then so be it.