I'm responding to what you said to me.
You posed the following:
In both of these hypotheticals, the younger person is specifically still in highschool. In the latter, they are dating someone with power and authority over them. In both of these situations, the age of the older party doesn't matter at all. If someone's prom date was 21, it would be just as strange as them being 30. A 22-year-old who got their position teaching high school right after they graduated college wouldn't be allowed to date a student, despite the relatively small age gap.
But here's the real point: is the objection about lived experience, or fundamental childishness of people under a certain age?
If the former, then relative age should never stop mattering. An significantly older person would always have more hours logged in life, even if their individual experiences are highly variable.
If it's the latter, then I can't argue with how you've judged the situation, based on your framework of thought. If you think the brain or physical development of someone under, say, 21 makes them - for all intents and purposes - still a child, then I can't argue with you judging the situation described in the OP as problematic.
So, which is it?
Okay, clearly my hypothetical about a teacher/student relationship is a bad one because as you say, a teacher would not be able to date their Highschool student. The intent was to describe that their is a big difference between where a person would be at 18-19 as someone a decade removed from highschool. So in that example the 18-year old would have graduated and then went in to date the teacher who was around 30. Still, not a very good example nonetheless.
I also do not agree with you about 21 and 18 year old being the same as a 30 year old and 18 year old when it comes to prom. In the former the two could have been dating while in school together or grew up together. There is no potential for a prior in school or out of school relationship between the latter without it being completely illegal. If my daughter went to prom with a 21 year old but I've known him and his parents for many years and they were close during school then I wouldn't be so troubled. There would be shared life experience in their teenage years. I would have to view that differently.
Now random 21 year old I've never met? Yeah that's a different discussion. However there is even a big difference between 21 and 30. I view 21 and 18 differently then I would view 18 and 30.
As for the question of "which is it?". I would say I definitely lean more towards the latter, however I also do not agree with the former because life experience of an 18 year and 28 year old is a world apart from lived experience between a 26 year old and 35 year old. As you age that line narrows even more.
Being 18-19 you are still very immature and haven't really been fully thrust into the world. I'm no brain expert nor will claim to be, so I can only go based on my experience. I didn't know anything at 18-19, even if I thought I did. Who I was at 28 was not the same as who I was in Highschool or right after graduating Highschool. In my case I was a father, had a job for years and in the process of purchasing a home. Today at 37 I've learned more as a parent, but enough to say that 37 year old me and 28 year old me would feel wrong? No, I would not. I make more money than I did back then, but outside of being a parent not much has changed with me a person from then to now. Not as much as 18 and 28.
So I guess yes, I would have to say I lean toward the latter.
I'm sure I rambled on quite a bit. Trying best to out my thoughts into words for this one.