
Because her hair is still short. It will slowly grow longer. And probably to what she been through in her timeline, it might had affected her growth. Don't forget that she been locked up for years since a kid. Pumped full of meds. And all she knows are stuff she reads in books. Never going out until the day she escaped. They all have a lot to learn and experience new stuff & emotions.

She is over 18. She been locked up for more than 10 years. She even almost lost all of her hair. Didn't feed her well either. Without all the nutrients, you can't grow up taller and stronger.
Though, some people are really small for their age. Like my middle school classmates, girls were around age 12 & 13, she looked like 8 or 9 with her height ( ̄∇ ̄")
Besides, don't forget that Beastmen are taller than average human being. Even you might look smaller under mister bear (≧∀≦)
how old is she she look like a lil kid they have to be pedos and you cant tell me otherwise wise cuz she aint even close to being over 18 less and if she is thats the biggest cap and she look like she just turned 12