Yuki is very childish, immature, wears oversized clothing, acts and speaks like a toddler, names everyday objects with toddler names, wants everyone should drop what they're doing to take care of her and prioritize her over anything else, acts like she can't do anything for herself even though she's a perfectly capable person, speaks in broken syntax, is a general attention seeker, throws tantrums when she doesn't get her way, blah blah blah. She is the same age as the other students at Hope's Peak Academy, yet she acts immature. She couldn't be allowed to be in the class meant for her age because of her childish immaturity, no matter how many times her father tried to get her in (her mother is trying to not let her end up in that class because Yuki's behavior is immature, even if Yuki thinks her mother is being rude when in reality Monokuma told her mother she's not in that class).