It seems to me that as soon as you get your ID you are ok to non rev. You'll need to be cass to ride in the cockpits of other companies airplanes in most instances, trust me if a seat is available in the back you will probably want that. The first couple of times in something you don't fly is cool up front though.

Just remember if you are on someone else’s (company) airplane it really is a privilege, respect it.