I don't get it, was the top still a child when mc got a crush on him?

The instructor at the time was 20 and Jaeha was 17, as I understood it.

and i also dont think he knew who exactly was sending the letters. he just knew it was a sunbae from his dojo, started to love them from their words, and wanted to follow up someday. jaeha had a one way love for several years since he first saw him, meanwhile mc had no idea it was him he was falling in love with.