My adhd son repeated 9th grade. It was a good decision as he needed the extra time – it took that extra year, plus all 4 high school years to figure out how to function with his adhd.
Many (all?) kids with adhd are a few years behind their peers in the brain development area. Not necessarily maturity as it is commonly thought of. My son was very responsible, e.g., but his interests were those of someone a few years younger.