Late reply I know, but maybe her particularly short body also doesn't shown signs of aging as much?
Though I still find it hard to believe people were mistaking her for a baby, her height would make her a toddler (2-3 year old) at best.
I guess with the infantile clothing, diaper, babbling/drooling, and feeding etc it made up for that.
Edit: I still enjoyed the story though to be clear, wish that fictionally strong hypnosis existed like that.