What's the youngest a 18, 19, 20 year old could date without it being weird?

I constantly think about this because when you apply the rule at 18 (divide by 2 and add 7) the gap is narrow. But I know plenty of people who's parents have a way larger age gap. And sure it obviously wouldn't be okay to date an elementary school student but where is the line drawn? I just want to know more because I've thought about it since I was about 17.