While a student pilot, I ran into this situation with a test Proctor who would not allow me to take the Commercial Written with an endorsement that was more than 24 months old. While I agree, and pointed out the FAA does not have a reg for this, the Proctor made me get a new endorsement. I can only speak from my experiences, some Proctors won't let you take the writtens. BTW, ATP makes you become a test Proctor for CATS and we never discussed this in the Proctor Class. The only thing we discussed was the expiration of knowledge exams after 24 calender months, and the exception that Airline Transport Pilot written exam can be used for a practical test provided the applicant is working under part 121 as a crewmember.