Say there's a 25-year-old man who's known an 18-year-old woman since she was 14. He sort of saw her as a little sister, but probably had some romantic feelings for her mixed in. He didn't confess his feelings for her because he wasn't entirely sure how he felt about her, and felt it would be inappropriate for a 21-year-old man to tell a 14-year-old girl that he was in love with her. Still, they have a pretty close friendship.
Would it be creepy if, when he was 25 and she was 18, that developed into a romantic and sexual relationship? Would it matter who instigated the relationship? The man wasn't being nice to the girl in the hopes that she would start a relationship with him. In fact, he was a little colder to her than another girl her age whom he truly saw as like a little sister because he was afraid the girl he liked would be intimidated by any affection toward her.
Also, would the fact that this takes place in somewhere between the Middle Ages and the Enlightenment have any bearing on how creepy or not-creepy the situation is?