A more grounded and in-verse consistent justification could be that when Y/N’s quirk awakened, she was in a “coma” like state for a while (3 or so years) where her body stopped aging and she couldn’t move her body but she was still conscious and aware of her surroundings.
Would give angsty backstory wherein her mother and newborn baby brother are aware that they can speak to her and teach her things and whatever but she can’t reciprocate.
So she is mentally 18, she has lived for 18 years and experienced all of them but had 3 years of her life following her quirk awakening stolen from her, effectively stunting her academic, physical and social development to the level of her little brother Izuku.
Just an idea
