Even so, 12 years is a big age gap, when you're only 18.
Theoretically, it wouldn't be a crime for him to have a relationship with you, as long as nothing happened while he was your teacher. Now you're at uni, you are free to do anything you like with him.
Having said that, I would advise against it. It would look too good for him to be in a relationship with someone who is barely older than his pupils, especially if he first got to know that person in that capacity of a teacher. It may not be a crime, but it would certainly be frowned upon, and may affect his career should news get about. You ought to take your own welfare into account as well - how well do you really know this guy? I got on really well with several of my teachers, and have kept in touch, and feel I know them pretty well, but there's a big step between friendship and the kind of closeness I need to feel before I enter into a relationship with someone.