@roxanneayy 12. She was 11 in the beginning but since then has had a birthday party. They don't probably have an age difference of more than about 7 years since he ought to be only 19 now (he became a knight at 13, had worked for five years when Estelle asked at the beginning of the story, a year ago). However, it's still an adult and a young girl far from adulthood, so I find it dubious as well. If they were 16 and 23, I wouldn't think much of it considering the historical setting. But 12 and small for her age means Emiel would be a pedophile to find her attractive as a woman.
However, I'm still not entirely sure how Emiel exactly feels like. He acts very inappropriately for his position (and the duke for some reason doesn't care), but there is a possibility he's just psychologically damaged by his past and hasn't had too many solid relationships of any kind with anyone, so even though Estelle is just a brat, she might be his first friend and the first person who really needs him personally and appreciates him.