I read The Hobbit when I was like 5, maybe even 4, and I absolutely thought the elves were like Christmas elves because I had no other concept and if anything in the text contradicted that, it went over my head.
(cw: Harry Potter bellow)
spoiler
I thought Snape was a woman, but not just that, I thought he looked like a stereotypical Halloween witch. I was like TEN when I read The Sorceror's stone, and somehow I wasnt really disabused of this notion until the movie came out?
accidentally seeing sex scenes in movies when really young made me think that sex was when you made out with someone whilest naked until I was like ... twelve. Despite the fact that apparently my mom had a frank sex talk with me when I was really young that I dont even remember. Like the only difference in my head between making out and sex was if you had clothes on.
(cw: Harry Potter bellow)
spoiler
Elrond as a christmas elf is one hell of a mental image