The age doesn't really matters.

When is it unappropriated for a teacher to date a student?
Lets see. If he is a teacher in school A, e she is a student in school B, that is not problem at all.
If he is a teacher in school A and gives classes to her, i get where the ethical problem is.
If he is a teacher in school A, and she is a student in the same school, but they do not connect each other in any professional way, I don't a see a problem.
It is just a person with a job, being it a teaching job, and another person that is doing her studies, the two things just happen to be happening in the same place.