It's the cultural narrative of where adulthood begins these days.

Childhood has been extended through your 20s.

it used to be 18 in the 1950s.

My nephew is 16. He thinks he can't date a 15 year or its pedophilia. He thinks this because it's what all the other kids his age think and he is terrified of being called a pedo.

So when a girl who was a grade younger than him asked him out he said no, even though he really liked her, because he didn't want to get socially ostracized for breaking the rules. Awhile later a girl who is 16 flirted with him and he asked her out, but that's cool because they are the same age and there is no 'power imbalance'.

There is a lot of 'age paranoia' going on in online spaces that has filtered out into the real world, such that an 18 year old dating a 16 year old (which I did when I was in HS) is considered horrible and awful, not normal and acceptable.

And some folk extend this to various other ages and perceived 'age gaps'. Like 28 year old dating a 24 year old being considered 'predatory' now. It's stupid... but people believe lots of crazy/stupid things and you aren't going to convince them otherwise because they like using age gaps as some sort of measure of moral acceptability for other people's relationships.