The introduction of the new university system cancelled the obligation of compulsory attendance for students.
You can take an exam for the first time only after you have inserted the corresponding course in your Annual Personal Study Plan and only after the course has actually been delivered.  
You can sit an exam until the last academic year in which the corresponding course is offered: therefore, if a course is no longer delivered, you can take the exam until the last examination session of the year preceding  the deactivation (September).  After this deadline, if you have not passed the exam, you will have to substitute it for a currently active course indicated by the Collegio of your degree programme.