when can you enter an advanced career?

Some of them don't have restrictions but does it say anywhere in the rules what you have to do to enter an advanced career?

Some careers have specific requirements listed on the back of the career cards, but the main point of an advanced career is that it is one that cannot be chosen at character creation, assuming you are creating beginning level characters.

Other than that, an advanced career can be selected at any time, provided you meet any specific requirements.