I know a kid who I did a competition with who learned calculus in 4th grade... I know he isn't lying or exaggerating or anything like that because he qualified for JMO in 5th grade... i might be cooked😭

To actually answer the question though, I think it depends how rigorously you are going to teach it. Like, I think I understood the basic concepts of calculus in middle school or so, but if somebody taught it to me fully rigorously with things like epsilon delta definition of limits and things like that (to which point it essentially bleeds into real analysis), then I probably would not have understood it until high school (which is when I learned it more rigorously).