@onionskins
Imbalance of power. If you're really mature enough to date an older person, and if they really love you, then it's much more reasonable to wait a couple years.
Otherwise the older person is most likely just manipulating the teenage child.
Kids make the same mistakes together and are on a more equal footing, and even then highschool relationships mess people up. But at least it isn't done with the same level of malice/intent as an adult taking advantage of the difference of power they hold.
Sure, there's always exceptions but those are exceptionally rare and given the risk and the possibility of *ruining a child life* you're a shit person for taking the risk at all.
That's why Adults get arrested for it, but the kids don't.