The big issue that remains after 18 is power disparity. A 40 year old with a stable career, savings, a house, etc has way more power in the relationship than someone who is still in college and has no real resources of their own as an example.

It's really easy for a bad actor to isolate and abuse a younger partner to the point where it's near impossible for them to leave because they have nothing to fall back on. Doesn't mean it happens every time, but that's inherently why people get pretty leery of age gaps like that even after 18. Because they can be used to trap people in abusive relationships and leave them with little choice but to rely on their abuser.