Psychologically, your personality is set by the time you are 18 or thereabouts. Personality disorders, by definition, are diagnosed from having traits before this age. For Miesa, her maturation and growth are likely stunted from being repressed so long while as a child, so while her ability to learn "normal" things was there, it just wasn't used. She can change, but it'll be as an adult and we all know how much more consciously and slow it takes to work on those aspects at an older age.