A couple of my professors always signs off on their emails with their first name only, such as, "best, Simon".

Does this mean that I could write my emails to them with, "Dear Simon" or "Hi Simon" ...and also say to him in person "good morning, Simon"?

Or, should I stick with "Professor "?

Should I ask for permission, e.g., "can I call you Simon?" ?

(Not sure whether age matters, but both of these professors are very young...)