It's interesting that German has two words for different sorts of love. In English the single, overworked word “love” has to cover such a multiplicity of different emotional states. I think it would be useful if about a dozen different words were available to do the work!

Here's how I see the 'in love ever since Emma was thirteen' remark (Ch.53):

Mr Knightley has known Emma all her life. While he is a man in his 20s she is a little girl. When Emma is thirteen John Knightley marries Isabella, and Emma becomes Mr Knightley's sister-in-law – she is “family”. This is how I think he has her categorised right up to and including the early stages of the novel.

At the same time I think he has long been drawn to her intelligence and sparky personality. In Ch.5 we are told:

”At ten years old, she had the misfortune of being able to answer questions which puzzled her sister at seventeen. She was always quick and assured: Isabella slow and diffident. And ever since she was twelve, Emma has been mistress of the house and of you all.”

“Emma has been meaning to read more ever since she was twelve years old. I have seen a great many lists of her drawing up at various times of books that she meant to read regularly through ........ The list she drew up when only fourteen -- I remember thinking it did her judgment so much credit, that I preserved it some time;”(Ch.5) I think this comment indicates that he had a more than ordinary degree of liking and interest even then..

I think Emma's looks have also been a part of her charm since she reached physical maturity and quite possibly before. All in all, I think Emma has been a large part of the attraction which has caused Mr Knightley to form the habit of visiting Hartfield so frequently. Would he have called quite so often for the sake of Mr Woodhouse's sparkling conversation? Somehow I doubt it.

I don't see him struggling for years with an inappropriate passion, or even biding his time until she is an adult. The whole tone of the conversation between Mr Knightley and Mrs Weston in Ch.5 makes it clear to me that there is no question of him having 'improper' thoughts about Emma when she was only thirteen, or even at the start of the novel. Consider the way he says “I love to look at her” - it is so straightforward and open. His comment about wanting to see her in love and in some doubt of a return is also telling IMO. This would not have been said if he were 'in love' with her in an adult, man-woman sort of a way. Feelings of 'that' sort just haven't come into it so far. The liking and affection and pleasure in her looks have all been there, but because of the 'family' connection, the age difference and the long-established habits of their relationship, Emma has been in the wrong category to be the object of such feelings. It is like the sort of relationship one might have with a sibling, or perhaps a teacher and student. One can really like them, feel great affection for them, and admire and take pleasure in their looks. But the 'man-woman' type feelings simply are not there because the person is in the 'wrong' category so one just does not perceive them in that way.

Something that makes me smile about all this is that Emma is often characterised in discussion etc. as “clueless”. Perhaps she is, but the ever so wise and perceptive Mr Knightley has also been utterly clueless, probably for some years, about the nature of his feelings for Emma, right up to the point when the potential threat from Frank Churchill causes a reassessment. That has evidently happened by the time of the ball in Ch.38: