20? I'd say that as soon as they reach the age of consent (preferably a more Western style age of consent like 16 to 18, than China's excessively low one of 14), they aren't technically doing anything wrong, no? Yeah, they are naive and can make incorrect choices, but so are some of the younger male FTs in their early 20s that bonk them.

As for your own students, that's an abuse of trust issue. It is also not in your own interests, especially if you end up accidentally marking her for her oral skills when you are supposed to marking a written piece of coursework.

I actually made a student who was excessively friendly and in early 20s cry when I said something about the boundaries between teachers and students, but... she's not getting special treatment from me for being pretty, she's just the same as everybody else, a student who is there for the purpose of learning.

If I see her after leaving this job, that's different as she was quite a stunner, lol.